Correctly indicate invalid partition context

BUG=aomedia:685

Change-Id: I8490af713c53e8c420ef9dffa7c27bb71bbfca54

av1/common/enums.h

@@ -149,6 +149,7 @@ typedef char PARTITION_CONTEXT;
 #define PARTITION_BLOCK_SIZES (4 + CONFIG_EXT_PARTITION)
 #define PARTITION_CONTEXTS_PRIMARY (PARTITION_BLOCK_SIZES * PARTITION_PLOFFSET)
 #if CONFIG_UNPOISON_PARTITION_CTX
+#define INVALID_PARTITION_CTX (-1)
 #define PARTITION_CONTEXTS \
   (PARTITION_CONTEXTS_PRIMARY + 2 * PARTITION_BLOCK_SIZES)
 #else

av1/common/onyxc_int.h

@@ -913,7 +913,7 @@ static INLINE int partition_plane_context(const MACROBLOCKD *xd, int mi_row,
   else if (!has_rows && has_cols)
     return PARTITION_CONTEXTS_PRIMARY + PARTITION_BLOCK_SIZES + bsl;
   else
-    return PARTITION_CONTEXTS;  // Bogus context, forced SPLIT
+    return INVALID_PARTITION_CTX;  // Bogus context, forced SPLIT
 #else
   const PARTITION_CONTEXT *above_ctx = xd->above_seg_context + mi_col;
   const PARTITION_CONTEXT *left_ctx =