(xinput) Clear stale pad state when XInputGetState fails

Companion to fda77fa, which fixed the equivalent issue in dinput.

Both xinput_joypad.c and xinput_hybrid_joypad.c poll into the cached
state directly and only inspect the result to detect connect/disconnect
transitions:

   status        = g_XInputGetStateEx(i, &state->xstate);
   success       = status == ERROR_SUCCESS;
   new_connected = status != ERROR_DEVICE_NOT_CONNECTED;

   if (new_connected != state->connected)
   {
      ...
   }

'success' is computed but only ever read inside that branch, so when a
call fails for any reason other than ERROR_DEVICE_NOT_CONNECTED the pad
remains marked connected and 'state->xstate' keeps whatever was read
last. XInput can return ERROR_ACCESS_DENIED or ERROR_BUSY, and
transient failures occur during device arbitration - for instance when
another process takes exclusive access, or across a fast user switch.

xinput_joypad_button() gates only on state->connected, then reads
state->xstate.Gamepad.wButtons, which carries the XINPUT_GAMEPAD_DPAD_*
bits. A direction held at the time of the last good read therefore
continues to be reported for as long as the failure persists.

Read into a temporary and copy it into the cached state only on
success, zeroing the cache otherwise. Reading into a temporary also
avoids leaving the cached state indeterminate should the call fail
after partially writing the output buffer.

Unlike the dinput POV case, a plain memset is correct here:
XINPUT_GAMEPAD.wButtons uses zero to mean no buttons pressed, whereas
DirectInput's rgdwPOV[] uses zero to mean north.

winraw_joypad.c was checked and does not share this defect: it is
event-driven via WM_INPUT with RIDEV_INPUTSINK, so it continues to
receive reports while unfocused, and its cached hats[] hold converted
bitmasks where zero already means centred.

input/drivers_joypad/xinput_hybrid_joypad.c

@@ -1149,6 +1149,7 @@ static void xinput_joypad_poll(void)
    for (i = 0; i < 4; ++i)
    {
       DWORD status;
+      XINPUT_STATE tmp_xstate;
       bool success, new_connected;
       xinput_joypad_state *state;
        /* On UWP, controllers may become available after initialization.
@@ -1163,9 +1164,22 @@ static void xinput_joypad_poll(void)
 #endif
 
       state         = &g_xinput_states[i];
-      status        = g_XInputGetStateEx(i, &state->xstate);
+      /* Read into a temporary: XInputGetStateEx() writing directly
+       * into the cached state would leave it indeterminate if the
+       * call fails partway. */
+      status        = g_XInputGetStateEx(i, &tmp_xstate);
       success       = (status == ERROR_SUCCESS);
       new_connected = (status != ERROR_DEVICE_NOT_CONNECTED);
+
+      if (success)
+         state->xstate = tmp_xstate;
+      else
+         /* Any status other than ERROR_DEVICE_NOT_CONNECTED leaves
+          * the pad marked connected, so the cached state would keep
+          * reporting the buttons held at the last good read - see
+          * the matching comment in xinput_joypad.c */
+         memset(&state->xstate, 0, sizeof(state->xstate));
+
       if (new_connected != state->connected)
       {
          state->connected = new_connected;

input/drivers_joypad/xinput_joypad.c

@@ -575,6 +575,7 @@ static void xinput_joypad_poll(void)
    for (i = 0; i < 4; ++i)
    {
       DWORD status;
+      XINPUT_STATE tmp_xstate;
       bool success, new_connected;
       xinput_joypad_state *state;
       /* On UWP, controllers may become available after initialization.
@@ -589,10 +590,29 @@ static void xinput_joypad_poll(void)
 #endif
 
       state          = &g_xinput_states[i];
-      status         = g_XInputGetStateEx(i, &state->xstate);
+      /* Read into a temporary: XInputGetStateEx() writing directly
+       * into the cached state would leave it indeterminate if the
+       * call fails partway. */
+      status         = g_XInputGetStateEx(i, &tmp_xstate);
       success        = status == ERROR_SUCCESS;
       new_connected  = status != ERROR_DEVICE_NOT_CONNECTED;
 
+      if (success)
+         state->xstate = tmp_xstate;
+      else
+      {
+         /* The pad is still considered connected for any status
+          * other than ERROR_DEVICE_NOT_CONNECTED (access denied,
+          * busy, transient failures during device arbitration),
+          * so the cached state would otherwise keep reporting
+          * whatever was held at the time of the last good read -
+          * including the Gamepad.wButtons D-pad bits, which
+          * xinput_joypad_button() consumes with no staleness
+          * check. Treat an unreadable pad as having nothing
+          * pressed. */
+         memset(&state->xstate, 0, sizeof(state->xstate));
+      }
+
       if (new_connected != state->connected)
       {
          /* Normally, dinput handles device insertion/removal for us, but