avcodec/cngdec: Check skip_samples

Without this its possible to make the new decode API decode billions of samples
out of a empty input and never return to the caller before all samples have been
created and discarded.

Fixes: Timeout
Fixes: 2992/clusterfuzz-testcase-6649611793989632

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cngdec.c

@@ -23,6 +23,7 @@
 
 #include "libavutil/common.h"
 #include "libavutil/ffmath.h"
+#include "libavutil/intreadwrite.h"
 #include "avcodec.h"
 #include "celp_filters.h"
 #include "internal.h"
@@ -120,6 +121,11 @@ static int cng_decode_frame(AVCodecContext *avctx, void *data,
         }
     }
 
+    if (avctx->internal->skip_samples > 10 * avctx->frame_size) {
+        avctx->internal->skip_samples = 0;
+        return AVERROR_INVALIDDATA;
+    }
+
     if (p->inited) {
         p->energy = p->energy / 2 + p->target_energy / 2;
         for (i = 0; i < p->order; i++)