src/useradd.c: set_defaults(): Do not free(3) the result of asprintf(3) if it failed

src/useradd.c: set_defaults(): Do not free(3) the result of asprintf(3) if it failed

See asprintf(3):

RETURN VALUE
     When successful,  these  functions  return  the  number  of  bytes
     printed, just like sprintf(3).  If memory allocation wasn’t possi‐
     ble,  or  some other error occurs, these functions will return -1,
     and the contents of strp are undefined.

Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>