avcodec/webp: Fixes null pointer dereference

Fixes: 1470/clusterfuzz-testcase-minimized-5404421666111488
Fixes: 1472/clusterfuzz-testcase-minimized-5677426430443520
Fixes: 1875/clusterfuzz-testcase-minimized-5536474562822144

Approved-by: BBB

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

libavcodec/webp.c

@@ -1351,6 +1351,9 @@ static int vp8_lossy_decode_frame(AVCodecContext *avctx, AVFrame *p,
     if (ret < 0)
         return ret;
 
+    if (!*got_frame)
+        return AVERROR_INVALIDDATA;
+
     update_canvas_size(avctx, avctx->width, avctx->height);
 
     if (s->has_alpha) {