unpoison_partition_ctx: Move the fake context to the end

It's less invasive and helps make future changes easier.

Change-Id: I0f858462f14109ace4d76a31daeac4289c94bc6e
diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index 7caeb1c..67a587b 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -792,7 +792,7 @@
   else if (!has_rows && has_cols)
     return PARTITION_CONTEXTS_PRIMARY + PARTITION_BLOCK_SIZES + bsl;
   else
-    return -1;  // Bogus context, forced SPLIT
+    return PARTITION_CONTEXTS;  // Bogus context, forced SPLIT
 #else
   const PARTITION_CONTEXT *above_ctx = xd->above_seg_context + mi_col;
   const PARTITION_CONTEXT *left_ctx =