bfi: fix division by 0

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/bfi.c

@@ -150,7 +150,7 @@ static int bfi_read_packet(AVFormatContext * s, AVPacket * pkt)
             return ret;
 
         pkt->pts          = bfi->video_frame;
-        bfi->video_frame += ret / bfi->video_size;
+        bfi->video_frame += bfi->video_size ? ret / bfi->video_size : 1;
 
         /* One less frame to read. A cursory decrement. */
         bfi->nframes--;