mmsh: fix integer overflow in mmsh_seek()

Fixes CID703745
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mmsh.c

@@ -394,7 +394,7 @@ static int64_t mmsh_seek(URLContext *h, int64_t pos, int whence)
     MMSContext *mms   = &mmsh->mms;
 
     if(pos == 0 && whence == SEEK_CUR)
-        return mms->asf_header_read_size + mms->remaining_in_len + mmsh->chunk_seq * mms->asf_packet_len;
+        return mms->asf_header_read_size + mms->remaining_in_len + mmsh->chunk_seq * (int64_t)mms->asf_packet_len;
     return AVERROR(ENOSYS);
 }