input/drivers_hid/iohidmanager_hid: hoist !hid NULL-check above hid->slots dereference

iohidmanager_hid_device_add had a dead-code NULL-guard:

    for (i = 0; i < MAX_USERS; i++)
    {
       struct iohidmanager_hid_adapter *a =
          (struct iohidmanager_hid_adapter*)hid->slots[i].data;
       ...
    }

    if (!(adapter = (struct iohidmanager_hid_adapter*)calloc(
             1, sizeof(*adapter))))
       return;
    if (!hid)
       goto error;

The 'if (!hid) goto error' guard only fires after the
for-loop has already unconditionally dereferenced hid via
hid->slots[i].data.  On NULL hid the function segfaults at
the slots lookup; the later guard is unreachable.

=== Reachability ===

iohidmanager_hid_device_matched is the sole caller and fetches
hid via:

    iohidmanager_hid_t *hid =
       (iohidmanager_hid_t*) hid_driver_get_data();
    iohidmanager_hid_device_add(device, hid);

hid_driver_get_data() can return NULL in two known windows:

  1. Early init: IOHIDManager matching callbacks can fire
     before iohidmanager_hid_init's return stores the hid
     pointer in the global driver state.  A device plugged
     in (or already present) at startup triggers the race.

  2. Teardown: iohidmanager_hid_free nulls the global before
     tearing down IOKit subscriptions, so a matching-callback
     dispatched just before the unsubscribe fires with NULL
     hid.

Both windows are small but real - especially on systems where
lots of HID devices enumerate at startup (USB hubs, Bluetooth
adapters), the first window is reliably reachable.

=== Fix ===

Hoist the !hid check to the top of iohidmanager_hid_device_add,
immediately after the local variable declarations and the
device_location_id read.  Remove the now-redundant later guard.

On NULL hid we return early without ever touching hid->slots or
allocating an adapter - a cleaner outcome than the previous
segfault and a strictly simpler state to reason about than the
old two-stage check (which would have leaked an allocated
adapter through goto-error on NULL hid if the dereference
crash had somehow not happened first).

=== Adapter freshness on goto error ===

The error: label at line ~946 walks adapter->hats /
adapter->axes / adapter->buttons linked lists.  These reads are
safe because adapter is always calloc'd fresh (zero-filled)
before any goto error fires from this function: the four
earlier goto-error sites at the time of writing (IOHIDDeviceOpen
failure, element enumeration failures, etc.) all run after the
calloc has succeeded, so adapter is non-NULL.  No change needed
there.

=== Scope ===

Single-file change.  Behaviour under valid (non-NULL) hid is
unchanged.  Behaviour under NULL hid goes from segfault to
silent skip of the device-add - which matches the behaviour
when the same device has already been registered (the earlier
early-return at the top of the function).

Thread-safety: IOHIDManager callbacks run on the run loop
thread, typically the main thread.  No concurrent mutation of
hid itself is introduced by this change - we still rely on
the caller (hid_driver_get_data) to return a consistent
snapshot of the global.

input/drivers_hid/iohidmanager_hid.c

@@ -650,6 +650,19 @@ static void iohidmanager_hid_device_add(IOHIDDeviceRef device, iohidmanager_hid_
       started (by deterministic method). if so do not re-add the pad */
    uint32_t device_location_id = iohidmanager_hid_device_get_location_id(device);
 
+   /* Hoist the !hid check above the hid->slots[i] dereference
+    * below.  hid_driver_get_data() can return NULL if a device-
+    * match callback fires before iohidmanager_hid_init's
+    * assignment or after iohidmanager_hid_free's teardown.
+    * The existing 'if (!hid) goto error' guard at line ~669
+    * was dead code because the for-loop directly below this
+    * comment reads hid->slots[i].data unconditionally -
+    * NULL-deref on NULL hid happened before we ever reached
+    * the calloc.  Fix by bailing out now before any hid
+    * dereference. */
+   if (!hid)
+      return;
+
    for (i = 0; i < MAX_USERS; i++)
    {
       struct iohidmanager_hid_adapter *a = (struct iohidmanager_hid_adapter*)hid->slots[i].data;
@@ -666,8 +679,6 @@ static void iohidmanager_hid_device_add(IOHIDDeviceRef device, iohidmanager_hid_
 
    if (!(adapter = (struct iohidmanager_hid_adapter*)calloc(1, sizeof(*adapter))))
       return;
-   if (!hid)
-      goto error;
 
    adapter->handle     = device;
    adapter->locationId = device_location_id;