firstboot: fix segfault when --locale-messages= is passed without --locale=
\#0 __strcmp_evex () at ../sysdeps/x86_64/multiarch/strcmp-evex.S:295
No locals.
\#1 0x0000557444eb172b in process_locale () at ../src/firstboot/firstboot.c:342
etc_localeconf = 0x7ffd40217b80 "/root/root/etc/locale.conf"
locales = {0x0, 0x0, 0x0}
i = 0
r = <optimized out>
__PRETTY_FUNCTION__ = "process_locale"
__func__ = "process_locale"
\#2 0x0000557444eaff93 in run (argv=0x7ffd40217d98, argc=3) at ../src/firstboot/firstboot.c:1401
loop_device = 0x0
unlink_dir = 0x0
r = <optimized out>
loop_device = <optimized out>
unlink_dir = <optimized out>
r = <optimized out>
__func__ = <optimized out>
__PRETTY_FUNCTION__ = <optimized out>
enabled = <optimized out>
_error = <optimized out>
_level = <optimized out>
_e = <optimized out>
_level = <optimized out>
_e = <optimized out>
\#3 main (argc=3, argv=0x7ffd40217d98) at ../src/firstboot/firstboot.c:1432
r = <optimized out>
__PRETTY_FUNCTION__ = "main"
Fixes https://github.com/systemd/systemd/issues/25249
(cherry picked from commit 4c4a73ce068ef16cfe7ad07c7c3386ac1dbc58fe)