fixed uninitialized memory use in arithmetic effect

Bug: skia:
Change-Id: Ieb591db974d858566b35c1e61c1303051ed65f67
Reviewed-on: https://skia-review.googlesource.com/150907
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed