lavf/fifo: fix undefined behaviour in deinit when destroying mutex

Reviewed-by: Jan Sebechlebsky <sebechlebskyjan@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/fifo.c

@@ -73,6 +73,7 @@ typedef struct FifoContext {
     int restart_with_keyframe;
 
     pthread_mutex_t overflow_flag_lock;
+    int overflow_flag_lock_initialized;
     /* Value > 0 signals queue overflow */
     volatile uint8_t overflow_flag;
 
@@ -515,6 +516,7 @@ static int fifo_init(AVFormatContext *avf)
     ret = pthread_mutex_init(&fifo->overflow_flag_lock, NULL);
     if (ret < 0)
         return AVERROR(ret);
+    fifo->overflow_flag_lock_initialized = 1;
 
     return 0;
 }
@@ -601,6 +603,7 @@ static void fifo_deinit(AVFormatContext *avf)
     av_dict_free(&fifo->format_options);
     avformat_free_context(fifo->avf);
     av_thread_message_queue_free(&fifo->queue);
+    if (fifo->overflow_flag_lock_initialized)
         pthread_mutex_destroy(&fifo->overflow_flag_lock);
 }