Lock joysticks when adding gamepad mappings

Fixes https://github.com/libsdl-org/SDL/issues/8412

src/joystick/SDL_gamepad.c

@@ -1670,6 +1670,8 @@ int SDL_AddGamepadMappingsFromRW(SDL_RWops *src, SDL_bool freesrc)
     }
     line = buf;
 
+    SDL_LockJoysticks();
+
     PushMappingChangeTracking();
 
     while (line < buf + db_size) {
@@ -1702,6 +1704,8 @@ int SDL_AddGamepadMappingsFromRW(SDL_RWops *src, SDL_bool freesrc)
 
     PopMappingChangeTracking();
 
+    SDL_UnlockJoysticks();
+
     SDL_free(buf);
     return gamepads;
 }