Fixed SDL_GameControllerMappingForGUID() crashing if no more memory available.

The return value of SDL_malloc() was not checked and NULL therefore not handled.
NULL returned by SDL_GameControllerMapping()/SDL_GameControllerMappingForGUID()
now either means "no mapping" (as before) or "no memory" (just crashed before).

src/joystick/SDL_gamecontroller.c

@@ -735,6 +735,10 @@ SDL_GameControllerMappingForGUID(SDL_JoystickGUID guid)
         /* allocate enough memory for GUID + ',' + name + ',' + mapping + \0 */
         needed = SDL_strlen(pchGUID) + 1 + SDL_strlen(mapping->name) + 1 + SDL_strlen(mapping->mapping) + 1;
         pMappingString = SDL_malloc(needed);
+        if (!pMappingString) {
+            SDL_OutOfMemory();
+            return NULL;
+        }
         SDL_snprintf(pMappingString, needed, "%s,%s,%s", pchGUID, mapping->name, mapping->mapping);
     }
     return pMappingString;