libobs: Check effect mutex exists before destroying it

Fixes a crash if graphics system didn't finish intializing.

libobs/graphics/graphics.c

@@ -213,7 +213,8 @@ void gs_destroy(graphics_t *graphics)
 	}
 
 	pthread_mutex_destroy(&graphics->mutex);
-	pthread_mutex_destroy(&graphics->effect_mutex);
+	if (graphics->effect_mutex)
+		pthread_mutex_destroy(&graphics->effect_mutex);
 	da_free(graphics->matrix_stack);
 	da_free(graphics->viewport_stack);
 	da_free(graphics->blend_state_stack);