fix overflow in rgnbuilder

Bug: oss-fuzz:6956
Change-Id: I244e49d458eb78e0c6200fc3c147f0f67823f97f
Reviewed-on: https://skia-review.googlesource.com/114780
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed