pmpdec: Check for zero audio packets.

This fixes a division by 0.

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

libavformat/pmpdec.c

@@ -124,6 +124,10 @@ static int pmp_packet(AVFormatContext *s, AVPacket *pkt)
     if (pmp->cur_stream == 0) {
         int num_packets;
         pmp->audio_packets = avio_r8(pb);
+        if (!pmp->audio_packets) {
+            av_log_ask_for_sample(s, "0 audio packets\n");
+            return AVERROR_PATCHWELCOME;
+        }
         num_packets = (pmp->num_streams - 1) * pmp->audio_packets + 1;
         avio_skip(pb, 8);
         pmp->current_packet = 0;