Fix partition decoding for 4x4 block

Stop reading partition information at 4x4 block.

Change-Id: I2b33f5ad307aa3051a1b1e230b7f6953ec6cecc6

av1/decoder/decodeframe.c

@@ -1764,8 +1764,9 @@ static void decode_partition(AV1Decoder *const pbi, MACROBLOCKD *const xd,
 
   if (mi_row >= cm->mi_rows || mi_col >= cm->mi_cols) return;
 
-  partition =
-      read_partition(cm, xd, mi_row, mi_col, r, has_rows, has_cols, bsize);
+  partition = (n4x4_l2 == 0) ? PARTITION_NONE
+                             : read_partition(cm, xd, mi_row, mi_col, r,
+                                              has_rows, has_cols, bsize);
   subsize = subsize_lookup[partition][bsize];  // get_subsize(bsize, partition);
 
 #if CONFIG_PVQ