Avoid overflow computing reserve for aaclip

BUG=chromium:713764
Change-Id: I32c95157d5f2b21e9981a07092558a1f6294a463
Reviewed-on: https://skia-review.googlesource.com/55380
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed