Fix regression in GrXferProcessor
Fix regression in GrXferProcessor

Fix segfault introduced in https://codereview.chromium.org/1170553002/

BUG=skia:

Review URL: https://codereview.chromium.org/1167243002

HalCanary/skia-hc,rubenvb/skia,vanish87/skia,vanish87/skia,rubenvb/skia,pcwalton/skia,tmpvar/skia.cc,todotodoo/skia,HalCanary/skia-hc,nvoron23/skia,Jichao/skia,HalCanary/skia-hc,qrealka/skia-hc,shahrzadmn/skia,noselhq/skia,shahrzadmn/skia,tmpvar/skia.cc,nvoron23/skia,nvoron23/skia,vanish87/skia,Jichao/skia,pcwalton/skia,rubenvb/skia,tmpvar/skia.cc,noselhq/skia,pcwalton/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Jichao/skia,noselhq/skia,qrealka/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,noselhq/skia,aosp-mirror/platform_external_skia,ominux/skia,todotodoo/skia,google/skia,ominux/skia,Hikari-no-Tenshi/android_external_skia,ominux/skia,aosp-mirror/platform_external_skia,ominux/skia,nvoron23/skia,shahrzadmn/skia,HalCanary/skia-hc,pcwalton/skia,rubenvb/skia,todotodoo/skia,nvoron23/skia,Hikari-no-Tenshi/android_external_skia,google/skia,ominux/skia,todotodoo/skia,tmpvar/skia.cc,nvoron23/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,ominux/skia,vanish87/skia,shahrzadmn/skia,google/skia,HalCanary/skia-hc,todotodoo/skia,tmpvar/skia.cc,Jichao/skia,google/skia,vanish87/skia,Jichao/skia,qrealka/skia-hc,Jichao/skia,vanish87/skia,noselhq/skia,rubenvb/skia,qrealka/skia-hc,shahrzadmn/skia,HalCanary/skia-hc,qrealka/skia-hc,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,todotodoo/skia,google/skia,Jichao/skia,nvoron23/skia,pcwalton/skia,rubenvb/skia,tmpvar/skia.cc,vanish87/skia,todotodoo/skia,rubenvb/skia,pcwalton/skia,google/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,Jichao/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,ominux/skia,noselhq/skia,Hikari-no-Tenshi/android_external_skia,shahrzadmn/skia,aosp-mirror/platform_external_skia,ominux/skia,noselhq/skia,HalCanary/skia-hc,pcwalton/skia,todotodoo/skia,qrealka/skia-hc,tmpvar/skia.cc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,shahrzadmn/skia,nvoron23/skia,noselhq/skia,ominux/skia,qrealka/skia-hc,pcwalton/skia,vanish87/skia,google/skia,pcwalton/skia,aosp-mirror/platform_external_skia,google/skia,todotodoo/skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,noselhq/skia,nvoron23/skia,vanish87/skia,qrealka/skia-hc,Jichao/skia,shahrzadmn/skia,shahrzadmn/skia