[buffer] Add assertions of buffer sync status to message_impl

src/hb-buffer.cc

@@ -2116,8 +2116,16 @@ hb_buffer_set_message_func (hb_buffer_t *buffer,
 bool
 hb_buffer_t::message_impl (hb_font_t *font, const char *fmt, va_list ap)
 {
+  assert (!have_output || (out_info == info && out_len == idx));
+
+  message_depth++;
+
   char buf[100];
   vsnprintf (buf, sizeof (buf), fmt, ap);
-  return (bool) this->message_func (this, font, buf, this->message_data);
+  bool ret = (bool) this->message_func (this, font, buf, this->message_data);
+
+  message_depth--;
+
+  return ret;
 }
 #endif

src/hb-buffer.hh

@@ -556,15 +556,11 @@ struct hb_buffer_t
     if (likely (!messaging ()))
       return true;
 
-    message_depth++;
-
     va_list ap;
     va_start (ap, fmt);
     bool ret = message_impl (font, fmt, ap);
     va_end (ap);
 
-    message_depth--;
-
     return ret;
 #endif
   }