avcodec/wmaprodec: Check offset

Fixes: index 33280 out of bounds for type 'float [32768]'
Fixes: 18718/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA2_fuzzer-5635373899710464

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

libavcodec/wmaprodec.c

@@ -1803,6 +1803,11 @@ static int xma_decode_packet(AVCodecContext *avctx, void *data,
     ret = decode_packet(avctx, &s->xma[s->current_stream], s->frames[s->current_stream],
                         &got_stream_frame_ptr, avpkt);
 
+    if (got_stream_frame_ptr && s->offset[s->current_stream] >= 64) {
+        got_stream_frame_ptr = 0;
+        ret = AVERROR_INVALIDDATA;
+    }
+
     /* copy stream samples (1/2ch) to sample buffer (Nch) */
     if (got_stream_frame_ptr) {
         int start_ch = s->start_channel[s->current_stream];