Fix decimal128_fast scalblnd128f implementation by constructing new val

Author: mborland

include/boost/decimal/decimal128_fast.hpp

@@ -1370,7 +1370,7 @@ constexpr auto scalblnd128f(decimal128_fast num, long exp) noexcept -> decimal12
     }
     #endif
 
-    num.exponent_ = static_cast<decimal128_fast::exponent_type>(static_cast<long>(num.biased_exponent()) + exp);
+    num = decimal128_fast(num.significand_, num.biased_exponent() + exp, num.sign_);
 
     return num;
 }