avformat/flvdec: Fix left shift of 137 by 24 places cannot be represented in type int

Fixes: 3c857d4d90365731524716e6d051e43a/signal_sigsegv_7f4f59bcc29e_1386_20abd2c8e655cb9c75b24368e65fe3b1.flv

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

libavformat/flvdec.c

@@ -858,7 +858,7 @@ retry:
         orig_size =
         size = avio_rb24(s->pb);
         dts  = avio_rb24(s->pb);
-        dts |= avio_r8(s->pb) << 24;
+        dts |= (unsigned)avio_r8(s->pb) << 24;
         av_log(s, AV_LOG_TRACE, "type:%d, size:%d, last:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, last, dts, avio_tell(s->pb));
         if (avio_feof(s->pb))
             return AVERROR_EOF;