Use SkTPin in magnifier filter to avoid integer overflow with NaN

Bug: chromium:1054887
Change-Id: I792a416c7071da75460016574f9aef8832f47e9d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/273319
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed