Fix incorrect return value on missing glyph

The GetGlyph function returns a boolean indicating whether the glyph
exists in the font. However, we were testing the pointer, so the test
would always succeed even if the glyph was missing. This interfered with
fallback algorithms for rendering compatibility characters.

Bug: 15816880
Bug: 16856221
Change-Id: I95cd45bb8f5e8ddf74cf23e6e624fbf30172e00a