The GSS_ERROR(x) macro may expand to `(x & value)` on some
implementations, instead of `((x) & value)`.  This is the case on macOS,
which means that if we attempt to wrap an expression in that macro, like
`a = b`, then that would expand to `(a = b & value)`.

Since `&` has a higher precedence, this is not at all what we want, and
will set our result code to an incorrect value.  Evaluate the expression
then test it with `GSS_ERROR` independently to avoid this.