avcodec/wmaprodec: Check that the streams channels do not exceed the overall channels

Fixes: NULL pointer dereference
Fixes: 18075/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5708262036471808
Fixes: 18087/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5740627634946048

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

libavcodec/wmaprodec.c

@@ -437,7 +437,7 @@ static av_cold int decode_init(WMAProDecodeCtx *s, AVCodecContext *avctx, int nu
         av_log(avctx, AV_LOG_ERROR, "invalid number of channels per XMA stream %d\n",
                s->nb_channels);
         return AVERROR_INVALIDDATA;
-    } else if (s->nb_channels > WMAPRO_MAX_CHANNELS) {
+    } else if (s->nb_channels > WMAPRO_MAX_CHANNELS || s->nb_channels > avctx->channels) {
         avpriv_request_sample(avctx,
                               "More than %d channels", WMAPRO_MAX_CHANNELS);
         return AVERROR_PATCHWELCOME;