user-record: fix segfault when processing matchHostname field (#40979)

Fix a typo which causes a segfault when processing a user record
with `matchHostname` when it's an array instead of a simple string:

```
$ echo '{"userName":"crashhostarray","perMachine":[{"matchHostname":["host1","host2"],"locked":false}]}' | userdbctl -F -
Segmentation fault         (core dumped)

$ coredumpctl info
...
       Message: Process 1172301 (userdbctl) of user 1000 dumped core.

                Module libz.so.1 from rpm zlib-ng-2.3.3-1.fc43.x86_64
                Module libcrypto.so.3 from rpm openssl-3.5.4-2.fc43.x86_64
                Stack trace of thread 1172301:
                #0  0x00007fded7b3a656 __strcmp_evex (libc.so.6 + 0x159656)
                #1  0x00007fded7e95397 per_machine_hostname_match (libsystemd-shared-260.so + 0x295397)
                #2  0x00007fded7e955b5 per_machine_match (libsystemd-shared-260.so + 0x2955b5)
                #3  0x00007fded7e957c6 dispatch_per_machine (libsystemd-shared-260.so + 0x2957c6)
                #4  0x00007fded7e96c97 user_record_load (libsystemd-shared-260.so + 0x296c97)
                #5  0x000000000040572d display_user (/home/fsumsal/repos/@systemd/systemd/build/userdbctl + 0x572d)
                #6  0x00007fded7ea9727 dispatch_verb (libsystemd-shared-260.so + 0x2a9727)
                #7  0x000000000041077c run (/home/fsumsal/repos/@systemd/systemd/build/userdbctl + 0x1077c)
                #8  0x00000000004107ce main (/home/fsumsal/repos/@systemd/systemd/build/userdbctl + 0x107ce)
                #9  0x00007fded79e45b5 __libc_start_call_main (libc.so.6 + 0x35b5)
                #10 0x00007fded79e4668 __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x3668)
                #11 0x00000000004038d5 _start (/home/fsumsal/repos/@systemd/systemd/build/userdbctl + 0x38d5)
                ELF object binary architecture: AMD x86-64
```