Fix code that left shifts a negative value. This has undefined behavior.

tests/ClampRangeTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkRandom.h"
 #include "Test.h"
 #include "gradients/SkClampRange.h"
@@ skipping 64 matching lines @@
     test_range( ff(2), 0, 20);
 
     test_range(-10, 1, 20);
     test_range(10, -1, 20);
     test_range(-10, 3, 20);
     test_range(10, -3, 20);
 
     test_range(ff(1),  ff(16384),  100);
     test_range(ff(-1), ff(-16384), 100);
     test_range(ff(1)/2, ff(16384), 100);
-    // TODO(reed): skia:2481, fix whatever bug this is, then uncomment
-    //test_range(ff(1)/2, ff(-16384), 100);
+    test_range(ff(1)/2, ff(-16384), 100);
 
     SkRandom rand;
 
     // test non-overflow cases
     for (int i = 0; i < 1000000; i++) {
         SkFixed fx = rand.nextS() >> 1;
         SkFixed sx = rand.nextS() >> 1;
         int count = rand.nextU() % 1000 + 1;
         SkFixed dx = (sx - fx) / count;
         test_range(fx, dx, count);
     }
 
     // TODO(reed): skia:2481, fix whatever bug this is, then uncomment
     /*
     // test overflow cases
     for (int i = 0; i < 100000; i++) {
         SkFixed fx = rand.nextS();
         SkFixed dx = rand.nextS();
         int count = rand.nextU() % 1000 + 1;
         test_range(fx, dx, count);
     }
     */
 }