Harden degenerate gradient context handling

src/effects/gradients/Sk4fGradientBase.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef Sk4fGradientBase_DEFINED
 #define Sk4fGradientBase_DEFINED
 
@@ skipping 10 matching lines @@
 GradientShaderBase4fContext : public SkShader::Context {
 public:
     GradientShaderBase4fContext(const SkGradientShaderBase&,
                                 const ContextRec&);
 
     uint32_t getFlags() const override { return fFlags; }
 
     void shadeSpan(int x, int y, SkPMColor dst[], int count) override;
     void shadeSpan4f(int x, int y, SkPM4f dst[], int count) override;
 
+    bool isValid() const;
+
 protected:
     struct Interval {
         Interval(const Sk4f& c0, SkScalar p0,
                  const Sk4f& c1, SkScalar p1);
 
         bool isZeroRamp() const { return fZeroRamp; }
 
         SkPM4f   fC0, fDc;
         SkScalar fP0, fP1;
         bool     fZeroRamp;
@@ skipping 23 matching lines @@
     template <DstType dstType, ApplyPremul premul>
     void shadePremulSpan(int x, int y, typename DstTraits<dstType, premul>::Type[],
                          int count) const;
 
     template <DstType dstType, ApplyPremul premul, SkShader::TileMode tileMode>
     void shadeSpanInternal(int x, int y, typename DstTraits<dstType, premul>::Type[],
                            int count) const;
 };
 
 #endif // Sk4fGradientBase_DEFINED