Prevent access to invalid pointer

The commit added check to make sure no invalid memory access even when
the decoder instance is never initialized.

Change-Id: I4da343d0b3c78c27777ac7f5ce7688562c69f0c5

vp9/vp9_dx_iface.c

diff --git a/vp9/vp9_dx_iface.c b/vp9/vp9_dx_iface.c
index 7a5b786..6b92316 100644
--- a/vp9/vp9_dx_iface.c
+++ b/vp9/vp9_dx_iface.c
@@ -658,8 +658,10 @@
 
   if (corrupted) {
     VP9D_COMP *pbi = (VP9D_COMP *)ctx->pbi;
-    *corrupted = pbi->common.frame_to_show->corrupted;
-
+    if (pbi)
+      *corrupted = pbi->common.frame_to_show->corrupted;
+    else
+      return VPX_CODEC_ERROR;
     return VPX_CODEC_OK;
   } else {
     return VPX_CODEC_INVALID_PARAM;