Don't dereference ctx->priv if it hasn't been setup correctly.

vp8/vp8_dx_iface.c

diff --git a/vp8/vp8_dx_iface.c b/vp8/vp8_dx_iface.c
index 36a0b39..7e4fc0b 100644
--- a/vp8/vp8_dx_iface.c
+++ b/vp8/vp8_dx_iface.c
@@ -528,7 +528,7 @@
 
     done = 1;
 
-    if (ctx->priv->alg_priv)
+    if (!res && ctx->priv->alg_priv)
     {
         for (i = 0; i < NELEMENTS(vp8_mem_req_segs); i++)
         {