guard against masks that are too big to blur

Check that the calculation of the tmp buffer size will not
overflow an int.

Bug=skia:27758

Change-Id: Ie1def7a4bf81f04e3231bfc1bd590f24ba3f5f7f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/365487
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed