Remove undefined behavior

* Taking the negative of a signed negative is UB, but
  taking the negative of an unsigned is not.

src/ripple/basics/impl/Number.cpp

@@ -173,9 +173,9 @@ Number::normalize()
         return;
     }
     bool const negative = (mantissa_ < 0);
-    if (negative)
-        mantissa_ = -mantissa_;
     auto m = static_cast<std::make_unsigned_t<rep>>(mantissa_);
+    if (negative)
+        m = -m;
     while ((m < minMantissa) && (exponent_ > minExponent))
     {
         m *= 10;