libobs: Only warn when releasing non-NULL source

Only warn when the pointer provided to obs_source_release() is
non-NULL. In some custom usages of libobs, libobs may be freed before
OBS* smart pointers (like OBSSource) are destructed, leading to a
misleading warning if the pointers have already been cleared with
nullptr.

Previous behavior is basically:
- Clear OBS* pointers with nullptr
- Unload libobs
- Those smart pointers that were cleared will still call
obs_source_release() on destruct
- Warning appears

libobs/obs-source.c

@@ -756,7 +756,7 @@ void obs_source_addref(obs_source_t *source)
 
 void obs_source_release(obs_source_t *source)
 {
-	if (!obs) {
+	if (!obs && source) {
 		blog(LOG_WARNING, "Tried to release a source when the OBS "
 				  "core is shut down!");
 		return;