Harden degenerate gradient context handling

src/effects/gradients/SkGradientShaderPriv.h

Index: src/effects/gradients/SkGradientShaderPriv.h
diff --git a/src/effects/gradients/SkGradientShaderPriv.h b/src/effects/gradients/SkGradientShaderPriv.h
index f850994761e02eea77aac6e6516794a60c00a55f..61a44184ff27d006accd60dc6babdac4156398df 100644
--- a/src/effects/gradients/SkGradientShaderPriv.h
+++ b/src/effects/gradients/SkGradientShaderPriv.h
@@ -115,7 +115,6 @@ public:
         SkAutoMalloc fDynamicStorage;
     };
 
-public:
     SkGradientShaderBase(const Descriptor& desc, const SkMatrix& ptsToUnit);
     virtual ~SkGradientShaderBase();
 
@@ -159,6 +158,8 @@ public:
 
         uint32_t getFlags() const override { return fFlags; }
 
+        bool isValid() const;
+
     protected:
         SkMatrix    fDstToIndex;
         SkMatrix::MapXYProc fDstToIndexProc;
@@ -233,6 +234,16 @@ protected:
                                    SkColor* colorSrc, Rec* recSrc,
                                    int count);
 
+    template <typename T, typename... Args>
+    static Context* CheckedCreateContext(void* storage, Args&&... args) {
+        auto* ctx = new (storage) T(std::forward<Args>(args)...);
+        if (!ctx->isValid()) {
+            ctx->~T();
+            return nullptr;
+        }
+        return ctx;
+    }
+
 private:
     enum {
         kColorStorageCount = 4, // more than this many colors, and we'll use sk_malloc for the space