avcodec/webp: Fix null pointer dereference

Fixes: 1369/clusterfuzz-testcase-minimized-5048908029886464

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

libavcodec/webp.c

@@ -1342,6 +1342,8 @@ static int vp8_lossy_decode_frame(AVCodecContext *avctx, AVFrame *p,
     pkt.size = data_size;
 
     ret = ff_vp8_decode_frame(avctx, p, got_frame, &pkt);
+    if (ret < 0)
+        return ret;
     if (s->has_alpha) {
         ret = vp8_lossy_decode_alpha(avctx, p, s->alpha_data,
                                      s->alpha_data_size);