Silence ASAN int32 overflow warning

It's fine to overflow SK_MaxS32 by one, the subsequent cast ensures
correct clamping.  But we need to cast earlier in order to make ASAN
happy.

TBR=mtklein@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2013243002

Review-Url: https://codereview.chromium.org/2013243002
1 file changed