Harden degenerate gradient context handling

src/effects/gradients/SkGradientShader.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fLinearGradient.h"
 #include "SkGradientShaderPriv.h"
 #include "SkHalf.h"
@@ skipping 379 matching lines @@
 
     // now convert our colors in to PMColors
     unsigned paintAlpha = this->getPaintAlpha();
 
     fFlags = this->INHERITED::getFlags();
     if (shader.fColorsAreOpaque && paintAlpha == 0xFF) {
         fFlags |= kOpaqueAlpha_Flag;
     }
 }
 
+bool SkGradientShaderBase::GradientShaderBaseContext::isValid() const {
+    return fDstToIndex.isFinite();
+}
+
 SkGradientShaderBase::GradientShaderCache::GradientShaderCache(
         U8CPU alpha, bool dither, const SkGradientShaderBase& shader)
     : fCacheAlpha(alpha)
     , fCacheDither(dither)
     , fShader(shader)
 {
     // Only initialize the cache in getCache32.
     fCache32 = nullptr;
     fCache32PixelRef = nullptr;
 }
@@ skipping 1327 matching lines @@
             (*stops)[i] = stop;
             stop = i < outColors - 1 ? stop + random->nextUScalar1() * (1.f - stop) : 1.f;
         }
     }
     *tm = static_cast<SkShader::TileMode>(random->nextULessThan(SkShader::kTileModeCount));
 
     return outColors;
 }
 
 #endif