obs-ffmpeg: Fix a race condition in create_or_fetch_log_context

Another thread could be manipulating the active_log_contexts array while the current thread is trying to read it, resulting in an uninitialized memory crash as the da_push_back call was not protected by the mutex.

plugins/obs-ffmpeg/obs-ffmpeg.c

@@ -36,7 +36,6 @@ static struct log_context *create_or_fetch_log_context(void *context)
 		new_log_context = cached_log_contexts.array[cnt - 1];
 		da_pop_back(cached_log_contexts);
 	}
-	pthread_mutex_unlock(&log_contexts_mutex);
 
 	if (!new_log_context)
 		new_log_context = bzalloc(sizeof(struct log_context));
@@ -47,6 +46,8 @@ static struct log_context *create_or_fetch_log_context(void *context)
 
 	da_push_back(active_log_contexts, &new_log_context);
 
+	pthread_mutex_unlock(&log_contexts_mutex);
+
 	return new_log_context;
 }
 
@@ -141,4 +142,4 @@ void obs_module_unload(void)
 
 	da_free(active_log_contexts);
 	da_free(cached_log_contexts);
-}
+}