Fix to overflow in asan test (for vp8 encoder).

Change-Id: Ie67339ab3a16ce7669b5366a743812df93c52711
diff --git a/vpx/src/vpx_encoder.c b/vpx/src/vpx_encoder.c
index e0a29cb..778ed2f 100644
--- a/vpx/src/vpx_encoder.c
+++ b/vpx/src/vpx_encoder.c
@@ -149,6 +149,7 @@
         cfg++;
         dsf++;
       }
+      ctx--;
     }
   }