native_posix: Check pointer before de-referencing it

To avoid a Coverity warning (203449):
https://github.com/zephyrproject-rtos/zephyr/issues/18354



Initialize a pointer to NULL, and check it later before
de-referencing it.
Coverity could not see that posix_print_error_and_exit()
never returns even that it ends with exit()

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>