Fix overflow in STSD parser

Reset `sc->stsd_count` before parsing entries. This number doesn't get
reset, which means that multiple parse passes can increment it past the
`sc->extradata` array end and cause OOB writes.

libavformat/mov.c

@@ -3258,6 +3258,7 @@ fail:
             av_freep(&sc->extradata[j]);
     }
 
+    sc->stsd_count = 0;
     av_freep(&sc->extradata);
     av_freep(&sc->extradata_size);
     return ret;