Initialize aom_input_ctx to zero

This prevents writing uninitialized values to the output Y4M header
when decoding an OBU file that doesn't have framerate info at
container level.

Change-Id: I07ee50565df0b4cb19ddae93c5de11f85ea6fdc8

aomdec.c

diff --git a/aomdec.c b/aomdec.c
index c2b6d64..e2320ae 100644
--- a/aomdec.c
+++ b/aomdec.c
@@ -528,7 +528,7 @@
   unsigned char md5_digest[16];
 
   struct AvxDecInputContext input = { NULL, NULL, NULL };
-  struct AvxInputContext aom_input_ctx;
+  struct AvxInputContext aom_input_ctx = { 0 };
 #if CONFIG_WEBM_IO
   struct WebmInputContext webm_ctx;
   memset(&webm_ctx, 0, sizeof(webm_ctx));