Prevent degenerate linear gradient instantiation

If the point distance exceeds SkScalar, nasty things tend to happen.

R=reed@google.com
BUG=636194
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2234663002

Review-Url: https://codereview.chromium.org/2234663002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 710bde8..820c5b2 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -790,7 +790,7 @@
                                          SkShader::TileMode mode,
                                          uint32_t flags,
                                          const SkMatrix* localMatrix) {
-    if (!pts) {
+    if (!pts || !SkScalarIsFinite((pts[1] - pts[0]).length())) {
         return nullptr;
     }
     if (!valid_grad(colors, pos, colorCount, mode)) {