add more checks for computing clamp counts, remove dead code

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index b34c7964ee4770e23c44f92aafcd2295fdff784c..c7d845a1885b74b17323f1c8578612407ea3752e 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -149,6 +149,7 @@ void shadeSpan_linear_clamp(TileProc proc, SkGradFixed dx, SkGradFixed fx,
                             int toggle, int count) {
     SkClampRange range;
     range.init(fx, dx, count, 0, SkGradientShaderBase::kCache32Count - 1);
+    range.validate(count);
 
     if ((count = range.fCount0) > 0) {
         sk_memset32_dither(dstC,
@@ -332,6 +333,7 @@ void shadeSpan16_linear_clamp(TileProc proc, SkGradFixed dx, SkGradFixed fx,
                               int toggle, int count) {
     SkClampRange range;
     range.init(fx, dx, count, 0, SkGradientShaderBase::kCache32Count - 1);
+    range.validate(count);
 
     if ((count = range.fCount0) > 0) {
         dither_memset16(dstC,