Fix uninitialised bits after uncompressed header

A few bits of data were not zeroed because aom_wb_overwrite_literal
was being used instead of aom_wb_write_literal.

BUG=aomedia:1041

Change-Id: If9196551e99b6e5eeaefc3fe022088088f1dcd51
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 7d26aa4..c26e753 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3128,8 +3128,8 @@
     tg_params_wb = wb;
     saved_offset = wb.bit_offset;
     if (have_tiles) {
-      aom_wb_overwrite_literal(&wb, 3, n_log2_tiles);
-      aom_wb_overwrite_literal(&wb, (1 << n_log2_tiles) - 1, n_log2_tiles);
+      aom_wb_write_literal(&wb, 3, n_log2_tiles);
+      aom_wb_write_literal(&wb, (1 << n_log2_tiles) - 1, n_log2_tiles);
     }
 
     if (!use_compressed_header(cm)) {