fix ubsan warning

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-ASAN

Change-Id: Ic8c5d95fdb899cb5293b2bc456f61b1637ec3aed
Reviewed-on: https://skia-review.googlesource.com/17489
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed