allow div-by-zero in shadow utils
allow div-by-zero in shadow utils

Bug: oss-fuzz:6665
Change-Id: I5cbe02bf574d849340c85c0964d92ece45daac1f
Reviewed-on: https://skia-review.googlesource.com/129932
Commit-Queue: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Auto-Submit: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Reviewed-by: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,google/skia,google/skia,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,google/skia,google/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia