Properly retrieve memory block in SkTextUtils

Fixes a crash when calling SkTextUtils::GetPath with empty text
(or otherwise gets no glyphs to draw).

Change-Id: I11711df354be0045ab348bdd7e6cc0c01d3a4e90
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209200
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Martin Vejdarski <martin@mainframe.co.uk>