Roll HarfBuzz to 1.4.6

third_party/harfbuzz-ng/src/hb-buffer-serialize.cc

Index: third_party/harfbuzz-ng/src/hb-buffer-serialize.cc
diff --git a/third_party/harfbuzz-ng/src/hb-buffer-serialize.cc b/third_party/harfbuzz-ng/src/hb-buffer-serialize.cc
index 63a0f34669ae94aa4532e6711a73559ff6925249..85696c58962aa92955dc7ce39efbe3ea8d4378f7 100644
--- a/third_party/harfbuzz-ng/src/hb-buffer-serialize.cc
+++ b/third_party/harfbuzz-ng/src/hb-buffer-serialize.cc
@@ -311,6 +311,8 @@ hb_buffer_serialize_glyphs (hb_buffer_t *buffer,
   if (!buf_consumed)
     buf_consumed = &sconsumed;
   *buf_consumed = 0;
+  if (buf_size)
+    *buf = '\0';
 
   assert ((!buffer->len && buffer->content_type == HB_BUFFER_CONTENT_TYPE_INVALID) ||
 	  buffer->content_type == HB_BUFFER_CONTENT_TYPE_GLYPHS);