vorbisdec: avoid invalid memory access

This fixes some invalid memory access caused later in the function
by res_chan[] not being set for all channels. This happens when a
channel doesn't appear a submap. This change simply returns a
decoder error when this situation is detected.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/vorbisdec.c

@@ -1581,6 +1581,9 @@ static int vorbis_parse_audio_packet(vorbis_context *vc)
         ch_left -= ch;
     }
 
+    if (ch_left > 0)
+        return AVERROR_INVALIDDATA;
+
 // Inverse coupling
 
     for (i = mapping->coupling_steps - 1; i >= 0; --i) { //warning: i has to be signed