use SkLeftShift in case the first arg is negative

Bug: oss-fuzz:6117
Change-Id: I08521aaf4372a63428a1f663f4da78e72dfdf225
Reviewed-on: https://skia-review.googlesource.com/125344
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed