fix null-pointer deref in SkComposeShader::toString()
fix null-pointer deref in SkComposeShader::toString()

Review URL: https://codereview.chromium.org/1185893007

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