Fix a division by zero when multiplying 0 * y with SafeNumerics.

BUG=488302

Review URL: https://codereview.chromium.org/1142783002

Cr-Commit-Position: refs/heads/master@{#330357}

base/numerics/safe_math_impl.h

@@ -176,8 +176,8 @@ typename enable_if<std::numeric_limits<T>::is_integer&& std::numeric_limits<
                        T>::is_signed&&(sizeof(T) * 2 > sizeof(uintmax_t)),
                    T>::type
 CheckedMul(T x, T y, RangeConstraint* validity) {
-  // if either side is zero then the result will be zero.
-  if (!(x || y)) {
+  // If either side is zero then the result will be zero.
+  if (!x || !y) {
     return RANGE_VALID;
 
   } else if (x > 0) {

base/numerics/safe_numerics_unittest.cc

@@ -239,6 +239,9 @@ static void TestArithmetic(const char* dst, int line) {
   TEST_EXPECTED_VALUE(0, (CheckedNumeric<Dst>() * 1));
   TEST_EXPECTED_VALUE(1, (CheckedNumeric<Dst>(1) * 1));
   TEST_EXPECTED_VALUE(-2, (CheckedNumeric<Dst>(-1) * 2));
+  TEST_EXPECTED_VALUE(0, (CheckedNumeric<Dst>(0) * 0));
+  TEST_EXPECTED_VALUE(0, (CheckedNumeric<Dst>(-1) * 0));
+  TEST_EXPECTED_VALUE(0, (CheckedNumeric<Dst>(0) * -1));
   TEST_EXPECTED_VALIDITY(
       RANGE_OVERFLOW, CheckedNumeric<Dst>(DstLimits::max()) * DstLimits::max());