Avoid undefined behavior in FMA.

If the result is zero, the result is not denormal, so we do not need to
follow the denormal special case logic. The check
rounding_mask & ansAsUint would always be zero, so it would never do
anything, but the calculation of rounding_mask might have had undefined
behavior due to an out-of-range exponent.

modules/compiler/builtins/abacus/source/abacus_math/fma.cpp

@@ -432,7 +432,7 @@ T ABACUS_API __abacus_fma_unsafe(const T x, const T y, const T z) {
   const SignedType finalExpBiased =
       (((ansAsUint & exp_mask) >> mantissa_bits)) + 2 * ansExp;
 
-  if (finalExpBiased < 1) {
+  if (ans != 0 && finalExpBiased < 1) {
     // The answer is a denormal, find what bits will be cut off and see if it's
     // going to result in a RTE case.
     const UnsignedType rounding_bits = UnsignedType(0x1)