Free the +3 floppy controller on machine shutdown

specplus3_765_init() allocates a upd_fdc and stores it in a file-scope
pointer. specplus3_shutdown() returned 0 and did nothing else, so nothing
ever released it: each machine table build allocated a new one and
abandoned the previous.

    Direct leak of 320 byte(s) in 1 object(s) allocated from:
      upd_fdc_alloc_fdc fuse/peripherals/disk/upd_fdc.c:387
      specplus3_765_init fuse/machines/specplus3.c:151
      specplus3_init fuse/machines/specplus3.c:130
      machine_add_machine fuse/machine.c:123

Free it in the shutdown hook. Both the +3 and the +3e install that same
hook, so machine_end() calls it once per machine; clearing the pointer
makes the second call a no-op rather than a double free.

With this and the startup-manager registration fix, twenty consecutive
load/unload/load cycles complete with no leaks and no sanitiser reports at
all.

fuse/machines/specplus3.c

@@ -450,6 +450,15 @@ ui_drive_inserted( const ui_media_drive_info_t *drive, int new )
 int
 specplus3_shutdown( void )
 {
+  /* specplus3_765_init() allocates this once; nothing released it, so a
+     second machine table build leaked the previous one. Both the +3 and
+     the +3e install this hook, so machine_end() calls it twice and it has
+     to tolerate the second call. */
+  if( specplus3_fdc ) {
+    libspectrum_free( specplus3_fdc );
+    specplus3_fdc = NULL;
+  }
+
   return 0;
 }