use safemath::mull for buffer sizes

Bug:780104
Change-Id: Ic683abd9c7d15ebb01b6e5d40dbeb6e76f102eff
Reviewed-on: https://skia-review.googlesource.com/95760
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed