avformat/mov: Fix deallocation when MOVStreamContext failed to allocate

Fixes: 260813283176b57b3c9974fe284eebc3_signal_sigsegv_7ffff713351a_991_xtrem_e2_m64q15_a32sxx.3gp with memlimit of 262144

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -3973,6 +3973,9 @@ static int mov_read_close(AVFormatContext *s)
         AVStream *st = s->streams[i];
         MOVStreamContext *sc = st->priv_data;
 
+        if (!sc)
+            continue;
+
         av_freep(&sc->ctts_data);
         for (j = 0; j < sc->drefs_count; j++) {
             av_freep(&sc->drefs[j].path);