avoid signed-integer overflow, which is undefined behavior

UBSAN gave for test/unittest/itoatest.cpp:87:
runtime error: signed integer overflow: 4611686018427387904 * 2 cannot be represented in type 'long'

test/unittest/itoatest.cpp

@@ -84,6 +84,8 @@ static void Verify(void(*f)(T, char*), char* (*g)(T, char*)) {
                 VerifyValue<T>(Traits<T>::Negate(i + 1), f, g);
             }
             last = i;
+            if (i > static_cast<T>(std::numeric_limits<T>::max() / static_cast<T>(power)))
+                break;
             i *= power;
         } while (last < i);
     }