avcodec/utils: check skip_samples signedness

Fixes Ticket5528

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

libavcodec/utils.c

@@ -2346,7 +2346,7 @@ int attribute_align_arg avcodec_decode_audio4(AVCodecContext *avctx,
             skip_reason = AV_RL8(side + 8);
             discard_reason = AV_RL8(side + 9);
         }
-        if (avctx->internal->skip_samples && *got_frame_ptr &&
+        if (avctx->internal->skip_samples > 0 && *got_frame_ptr &&
             !(avctx->flags2 & AV_CODEC_FLAG2_SKIP_MANUAL)) {
             if(frame->nb_samples <= avctx->internal->skip_samples){
                 *got_frame_ptr = 0;