avcodec/webp: Always set pix_fmt

Fixes: out of array access
Fixes: 1434/clusterfuzz-testcase-minimized-6314998085189632
Fixes: 1435/clusterfuzz-testcase-minimized-6483783723253760

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

libavcodec/vp8.c

@@ -2550,6 +2550,8 @@ int vp78_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     enum AVDiscard skip_thresh;
     VP8Frame *av_uninit(curframe), *prev_frame;
 
+    av_assert0(avctx->pix_fmt == AV_PIX_FMT_YUVA420P || avctx->pix_fmt == AV_PIX_FMT_YUV420P);
+
     if (is_vp7)
         ret = vp7_decode_frame_header(s, avpkt->data, avpkt->size);
     else

libavcodec/webp.c

@@ -1334,9 +1334,8 @@ static int vp8_lossy_decode_frame(AVCodecContext *avctx, AVFrame *p,
     if (!s->initialized) {
         ff_vp8_decode_init(avctx);
         s->initialized = 1;
-        if (s->has_alpha)
-            avctx->pix_fmt = AV_PIX_FMT_YUVA420P;
     }
+    avctx->pix_fmt = s->has_alpha ? AV_PIX_FMT_YUVA420P : AV_PIX_FMT_YUV420P;
     s->lossless = 0;
 
     if (data_size > INT_MAX) {