Harden linear gradients

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 fdb6e59e1ca56e52a1450e1400bb07ce0286930a..3c4784702ab36cadf12544fd6fe961864b5fe9b0 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -774,6 +774,8 @@ static bool valid_grad(const SkColor colors[], const SkScalar pos[], int count,
 static void desc_init(SkGradientShaderBase::Descriptor* desc,
                       const SkColor colors[], const SkScalar pos[], int colorCount,
                       SkShader::TileMode mode, uint32_t flags, const SkMatrix* localMatrix) {
+    SkASSERT(colorCount > 1);
+
     desc->fColors       = colors;
     desc->fPos          = pos;
     desc->fCount        = colorCount;