Harden degenerate gradient context handling

src/effects/gradients/Sk4fGradientBase.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fGradientBase.h"
 
 #include <functional>
@@ skipping 140 matching lines @@
 
     if (shader.fColorsAreOpaque && this->getPaintAlpha() == SK_AlphaOPAQUE) {
         fFlags |= kOpaqueAlpha_Flag;
     }
 
     fColorsArePremul =
         (shader.fGradFlags & SkGradientShader::kInterpolateColorsInPremul_Flag)
         || shader.fColorsAreOpaque;
 }
 
+bool SkGradientShaderBase::
+GradientShaderBase4fContext::isValid() const {
+    return fDstToPos.isFinite();
+}
+
 void SkGradientShaderBase::
 GradientShaderBase4fContext::buildIntervals(const SkGradientShaderBase& shader,
                                             const ContextRec& rec, bool reverse) {
     // The main job here is to build a specialized interval list: a different
     // representation of the color stops data, optimized for efficient scan line
     // access during shading.
     //
     //   [{P0,C0} , {P1,C1}) [{P1,C2} , {P2,c3}) ... [{Pn,C2n} , {Pn+1,C2n+1})
     //
     // The list may be inverted when requested (such that e.g. points are sorted
@@ skipping 252 matching lines @@
         fDc = DstTraits<dstType>::load(i->fDc);
     }
 
     const Interval* fFirstInterval;
     const Interval* fLastInterval;
     const Interval* fInterval;
     SkScalar        fPrevT;
     Sk4f            fCc;
     Sk4f            fDc;
 };