avcodec/wmaprodec: Check if the channel sum of all internal contexts match the external

Fixes: NULL pointer dereference
Fixes: 18689/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5715114640015360

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

libavcodec/wmaprodec.c

@@ -1930,6 +1930,8 @@ static av_cold int xma_decode_init(AVCodecContext *avctx)
         s->start_channel[i] = start_channels;
         start_channels += s->xma[i].nb_channels;
     }
+    if (start_channels != avctx->channels)
+        return AVERROR_INVALIDDATA;
 
     return ret;
 }