ipmovie: fix FPE

Fixed Ticket807
Bug found by: Oana Stratulat

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

libavformat/ipmovie.c

@@ -115,7 +115,7 @@ static int load_ipmovie_packet(IPMVEContext *s, AVIOContext *pb,
 
     int chunk_type;
 
-    if (s->audio_chunk_offset) {
+    if (s->audio_chunk_offset && s->audio_channels && s->audio_bits) {
 
         /* adjust for PCM audio by skipping chunk header */
         if (s->audio_type != CODEC_ID_INTERPLAY_DPCM) {