Harden degenerate gradient context handling

src/effects/gradients/Sk4fGradientBase.cpp

Index: src/effects/gradients/Sk4fGradientBase.cpp
diff --git a/src/effects/gradients/Sk4fGradientBase.cpp b/src/effects/gradients/Sk4fGradientBase.cpp
index a22136831d6ec1f233bd4bb0177d2df92e2957bb..8994db3a91f9bf4d9e5c3ee0592ec1354e1d62cd 100644
--- a/src/effects/gradients/Sk4fGradientBase.cpp
+++ b/src/effects/gradients/Sk4fGradientBase.cpp
@@ -158,6 +158,11 @@ GradientShaderBase4fContext::GradientShaderBase4fContext(const SkGradientShaderB
         || shader.fColorsAreOpaque;
 }
 
+bool SkGradientShaderBase::
+GradientShaderBase4fContext::isValid() const {
+    return fDstToPos.isFinite();
+}
+
 void SkGradientShaderBase::
 GradientShaderBase4fContext::buildIntervals(const SkGradientShaderBase& shader,
                                             const ContextRec& rec, bool reverse) {