SDL_SetHintWithPriority: actually, this test is good.

Otherwise, we end up calling the callback if both the old and new value
are NULL.

This reverts commit 5f25a6ea78605f9f5b600015b7532a06df3f02e2 (and also
trims some whitespace).

src/SDL_hints.c

@@ -66,7 +66,7 @@ SDL_SetHintWithPriority(const char *name, const char *value,
             if (priority < hint->priority) {
                 return SDL_FALSE;
             }
-            if (!hint->value || !value || SDL_strcmp(hint->value, value) != 0) {
+            if (((hint->value == NULL) != (value == NULL)) || (value && (SDL_strcmp(hint->value, value) != 0))) {
                 for (entry = hint->callbacks; entry; ) {
                     /* Save the next entry in case this one is deleted */
                     SDL_HintWatch *next = entry->next;