Don't draw strikes that are too small

This may fix a fuzzer bug that trips an
assert for empty rectangles. I could not reproduce
the fuzzer problem.

Bug: chromium:1029831
Change-Id: I88befb7c27d9ddc357e0a9e494e68125f8252f43
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257676
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>