Update test to check buffer state

Fix the logic detecting an empty picture which breaks the buffer
state. The state of the readBuffer after reading the glyph should
be valid.

Bug: skia:14163

Change-Id: I850eb2fedd3eadde17670eea6a6124cf5f1380f1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/658918
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed