avformat/aiffdec: Check for previously set channels

Fixes: out of array access (av_channel_layout_copy())
Fixes: 67087/clusterfuzz-testcase-minimized-ffmpeg_dem_AIFF_fuzzer-4920720268263424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aiffdec.c

@@ -107,6 +107,8 @@ static int get_aiff_header(AVFormatContext *s, int64_t size,
         size++;
     par->codec_type = AVMEDIA_TYPE_AUDIO;
     channels = avio_rb16(pb);
+    if (par->ch_layout.nb_channels && par->ch_layout.nb_channels != channels)
+        return AVERROR_INVALIDDATA;
     par->ch_layout.nb_channels = channels;
     num_frames = avio_rb32(pb);
     par->bits_per_coded_sample = avio_rb16(pb);