avoid overflow by using tmp 64bits

Bug: oss-fuzz:7746
Cq-Include-Trybots: skia.primary:Test-Debian9-Clang-GCE-CPU-AVX2-x86_64-Release-All-SKNX_NO_SIMD
Change-Id: Iba3ea50331fa06ca201ac316e33d4f79ab8ef459
Reviewed-on: https://skia-review.googlesource.com/121802
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed