avformat/flvdec: Check size at the top of the main loop

Fixes: signed integer overflow
Fixes: ffmpeg_flv_read_packet_libavformat_flvdec.c_1900_26.poc

Found-by: iceray-Li
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -1565,6 +1565,10 @@ skip:
 
     for (;;) {
         int track_size = size;
+        if (size < 0) {
+            ret = FFERROR_REDO;
+            goto leave;
+        }
 
         if (multitrack_type != MultitrackTypeOneTrack) {
             track_size = avio_rb24(s->pb);