avcodec/wmaprodec: Check if there is a stream

Fixes: null pointer dereference
Fixes: signed integer overflow: 512 * 2147483647 cannot be represented in type 'int'
Fixes: 17809/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5634409947987968

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

libavcodec/wmaprodec.c

@@ -1902,7 +1902,9 @@ static av_cold int xma_decode_init(AVCodecContext *avctx)
     }
 
     /* encoder supports up to 64 streams / 64*2 channels (would have to alloc arrays) */
-    if (avctx->channels > XMA_MAX_CHANNELS || s->num_streams > XMA_MAX_STREAMS) {
+    if (avctx->channels > XMA_MAX_CHANNELS || s->num_streams > XMA_MAX_STREAMS ||
+        s->num_streams <= 0
+    ) {
         avpriv_request_sample(avctx, "More than %d channels in %d streams", XMA_MAX_CHANNELS, s->num_streams);
         return AVERROR_PATCHWELCOME;
     }