avcodec/wmaprodec: Check that the EOF frame was allocated before decoding into it

Fixes: NULL pointer dereference
Fixes: 38125/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5151909422432256

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

@@ -1855,7 +1855,7 @@ static int xma_decode_packet(AVCodecContext *avctx, void *data,
         eof = 1;
 
         for (i = 0; i < s->num_streams; i++) {
-            if (!s->xma[i].eof_done) {
+            if (!s->xma[i].eof_done && s->frames[i]->data[0]) {
                 ret = decode_packet(avctx, &s->xma[i], s->frames[i],
                                     &got_stream_frame_ptr, avpkt);
             }