Initialization to prevent asan failure on google3

Reinstates an assignment to prevent an asan failurere on google3.
Not sure why the failure happens. This was removed in a recent patch
https://gerrit.chromium.org/gerrit/#/c/71068/.

Change-Id: Ifd9ccffd4c2164f4de38b21821ffb28bd779b0f3

vpxdec.c

@@ -538,7 +538,7 @@ int main_loop(int argc, const char **argv_) {
   struct VpxDecInputContext input = {NULL, NULL};
   struct VpxInputContext vpx_input_ctx;
 #if CONFIG_WEBM_IO
-  struct WebmInputContext webm_ctx;
+  struct WebmInputContext webm_ctx = {0};
   input.webm_ctx = &webm_ctx;
 #endif
   input.vpx_input_ctx = &vpx_input_ctx;