Fix signed/unsigned mismatch in shaper assert.

Change-Id: I05e0e3f5940281b8c9fdcbabf6d8c1013bfa48d4
Reviewed-on: https://skia-review.googlesource.com/c/192824
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>