libwebpenc_animencoder: zero initialize the WebPAnimEncoderOptions struct

This zeroes the WebPAnimEncoderOptions.verbose field, silencing library info messages
printed to stderr.

Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 626b6b769c)

libavcodec/libwebpenc_animencoder.c

@@ -41,7 +41,7 @@ static av_cold int libwebp_anim_encode_init(AVCodecContext *avctx)
     int ret = ff_libwebp_encode_init_common(avctx);
     if (!ret) {
         LibWebPAnimContext *s = avctx->priv_data;
-        WebPAnimEncoderOptions enc_options;
+        WebPAnimEncoderOptions enc_options = { 0 };
         WebPAnimEncoderOptionsInit(&enc_options);
         // TODO(urvang): Expose some options on command-line perhaps.
         s->enc = WebPAnimEncoderNew(avctx->width, avctx->height, &enc_options);