Avoid null cgFont crash in harfBuzzCoreTextGetTable

Source/platform/fonts/harfbuzz/HarfBuzzFaceCoreText.cpp

 /*
  * Copyright (c) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 91 matching lines @@
 
 static void releaseTableData(void* userData)
 {
     CFDataRef cfData = reinterpret_cast<CFDataRef>(userData);
     CFRelease(cfData);
 }
 
 static hb_blob_t* harfBuzzCoreTextGetTable(hb_face_t* face, hb_tag_t tag, void* userData)
 {
     CGFontRef cgFont = reinterpret_cast<CGFontRef>(userData);
+    if (!cgFont)
+        return 0;
     CFDataRef cfData = CGFontCopyTableForTag(cgFont, tag);
     if (!cfData)
         return 0;
 
     const char* data = reinterpret_cast<const char*>(CFDataGetBytePtr(cfData));
     const size_t length = CFDataGetLength(cfData);
     if (!data || !length)
         return 0;
     return hb_blob_create(data, length, HB_MEMORY_MODE_READONLY, reinterpret_cast<void*>(const_cast<__CFData*>(cfData)), releaseTableData);
 }
@@ skipping 13 matching lines @@
     hb_font_set_funcs(font, harfBuzzCoreTextGetFontFuncs(), m_platformData, 0);
     const float size = m_platformData->m_textSize;
     hb_font_set_ppem(font, size, size);
     const int scale = (1 << 16) * static_cast<int>(size);
     hb_font_set_scale(font, scale, scale);
     hb_font_make_immutable(font);
     return font;
 }
 
 } // namespace blink