change SkPoint::setLength to set itself to (0,0) if it starting length is degenerate

tests/PointTest.cpp

Index: tests/PointTest.cpp
diff --git a/tests/PointTest.cpp b/tests/PointTest.cpp
index fed443a02e7d050fc0b0b1d752c4abe8ed58875e..08ce7208eb007d25ce93696aa29cd75cb5b7cc37 100644
--- a/tests/PointTest.cpp
+++ b/tests/PointTest.cpp
@@ -108,13 +108,13 @@ static void test_overflow(skiatest::Reporter* reporter) {
 // report failure if we try to normalize them.
 static void test_underflow(skiatest::Reporter* reporter) {
     SkPoint pt = { 1.0e-37f, 1.0e-37f };
-    SkPoint copy = pt;
+    const SkPoint empty = { 0, 0 };
 
     REPORTER_ASSERT(reporter, 0 == SkPoint::Normalize(&pt));
-    REPORTER_ASSERT(reporter, pt == copy);  // pt is unchanged
+    REPORTER_ASSERT(reporter, pt == empty);
 
     REPORTER_ASSERT(reporter, !pt.setLength(SK_Scalar1));
-    REPORTER_ASSERT(reporter, pt == copy);  // pt is unchanged
+    REPORTER_ASSERT(reporter, pt == empty);
 }
 
 DEF_TEST(Point, reporter) {