avformat: Make avformat_free_context handle NULL

Work as the other free()-like functions.

Bug-Id: CID 1087081
CC: libav-stable@libav.org

libavformat/utils.c

@@ -2447,6 +2447,9 @@ void avformat_free_context(AVFormatContext *s)
     int i, j;
     AVStream *st;
 
+    if (!s)
+        return;
+
     av_opt_free(s);
     if (s->iformat && s->iformat->priv_class && s->priv_data)
         av_opt_free(s->priv_data);