verify that glyph position rect is non-empty in GrAtlasTextBlob::appendGlyph

BUG=skia:

Change-Id: I35d170480cb3320a9d116e34aaf740aa201a294f
Reviewed-on: https://skia-review.googlesource.com/7700
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed