Harden degenerate gradient context handling

src/effects/gradients/SkGradientShader.cpp

Index: src/effects/gradients/SkGradientShader.cpp
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 4ee82037d12104d3fd4f28181719ea6743476637..17302d9d4f36e736344635a58b2b6337f4a6b33f 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -397,6 +397,10 @@ SkGradientShaderBase::GradientShaderBaseContext::GradientShaderBaseContext(
     }
 }
 
+bool SkGradientShaderBase::GradientShaderBaseContext::isValid() const {
+    return fDstToIndex.isFinite();
+}
+
 SkGradientShaderBase::GradientShaderCache::GradientShaderCache(
         U8CPU alpha, bool dither, const SkGradientShaderBase& shader)
     : fCacheAlpha(alpha)