Fixed NULL pointer dereferencing in linked lists

libobs-opengl/gl-shader.c

@@ -673,7 +673,8 @@ void gs_program_destroy(struct gs_program *program)
 
 	if (program->next)
 		program->next->prev_next = program->prev_next;
-	*program->prev_next = program->next;
+	if (program->prev_next)
+		*program->prev_next = program->next;
 
 	glDeleteProgram(program->obj);
 	gl_success("glDeleteProgram");

libobs/media-io/audio-io.c

@@ -99,7 +99,8 @@ static inline void audio_output_removeline(struct audio_output *audio,
 		struct audio_line *line)
 {
 	pthread_mutex_lock(&audio->line_mutex);
-	*line->prev_next = line->next;
+	if (line->prev_next)
+		*line->prev_next = line->next;
 	if (line->next)
 		line->next->prev_next = line->prev_next;
 	pthread_mutex_unlock(&audio->line_mutex);

libobs/obs-display.c

@@ -86,7 +86,8 @@ void obs_display_destroy(obs_display_t *display)
 {
 	if (display) {
 		pthread_mutex_lock(&obs->data.displays_mutex);
-		*display->prev_next = display->next;
+		if (display->prev_next)
+			*display->prev_next = display->next;
 		if (display->next)
 			display->next->prev_next = display->prev_next;
 		pthread_mutex_unlock(&obs->data.displays_mutex);

libobs/obs.c

@@ -1326,7 +1326,8 @@ void obs_context_data_remove(struct obs_context_data *context)
 {
 	if (context && context->mutex) {
 		pthread_mutex_lock(context->mutex);
-		*context->prev_next = context->next;
+		if (context->prev_next)
+			*context->prev_next = context->next;
 		if (context->next)
 			context->next->prev_next = context->prev_next;
 		pthread_mutex_unlock(context->mutex);