userdb: fix a memory leak

userdb: fix a memory leak

[   49.275617] testsuite-46.sh[1862]: =================================================================
[   49.275870] testsuite-46.sh[1862]: ==1862==ERROR: LeakSanitizer: detected memory leaks
[   49.276039] testsuite-46.sh[1862]: Direct leak of 103 byte(s) in 14 object(s) allocated from:
[   49.276515] testsuite-46.sh[1862]:     #0 0x7f4dbc07243b in strdup (/lib64/libasan.so.8+0x7243b)
[   49.276707] testsuite-46.sh[1862]:     #1 0x7f4dbb3900d5 in free_and_strdup ../src/basic/string-util.c:952
[   49.276931] testsuite-46.sh[1862]:     #2 0x7f4dbb15c67d in json_dispatch_user_group_name ../src/shared/json.c:4699
[   49.277134] testsuite-46.sh[1862]:     #3 0x7f4dbb16da9b in json_dispatch ../src/shared/json.c:4395
[   49.277352] testsuite-46.sh[1862]:     #4 0x7f4dbb25b28e in userdb_on_query_reply ../src/shared/userdb.c:305
[   49.277603] testsuite-46.sh[1862]:     #5 0x7f4dbb2748b9 in varlink_dispatch_reply ../src/shared/varlink.c:760
[   49.277766] testsuite-46.sh[1862]:     #6 0x7f4dbb2748b9 in varlink_process ../src/shared/varlink.c:951
[   49.277975] testsuite-46.sh[1862]:     #7 0x7f4dbb27a001 in defer_callback ../src/shared/varlink.c:1897
[   49.278197] testsuite-46.sh[1862]:     #8 0x7f4dbb5d57dd in source_dispatch ../src/libsystemd/sd-event/sd-event.c:4191
[   49.278421] testsuite-46.sh[1862]:     #9 0x7f4dbb5d685d in sd_event_dispatch ../src/libsystemd/sd-event/sd-event.c:4780
[   49.278675] testsuite-46.sh[1862]:     #10 0x7f4dbb5d70bf in sd_event_run ../src/libsystemd/sd-event/sd-event.c:4841
[   49.278873] testsuite-46.sh[1862]:     #11 0x7f4dbb257e7c in userdb_process ../src/shared/userdb.c:591
[   49.279048] testsuite-46.sh[1862]:     #12 0x7f4dbb25f78f in membershipdb_iterator_get ../src/shared/userdb.c:1411
[   49.279280] testsuite-46.sh[1862]:     #13 0x7f4dbb23a98c in user_record_show ../src/shared/user-record-show.c:187
[   49.279504] testsuite-46.sh[1862]:     #14 0x404ae3 in show_user ../src/userdb/userdbctl.c:93
[   49.279710] testsuite-46.sh[1862]:     #15 0x40b4f5 in display_user ../src/userdb/userdbctl.c:418
[   49.279961] testsuite-46.sh[1862]:     #16 0x7f4dbb2804d2 in dispatch_verb ../src/shared/verbs.c:110
[   49.280233] testsuite-46.sh[1862]:     #17 0x40dcf3 in run ../src/userdb/userdbctl.c:1327
[   49.280434] testsuite-46.sh[1862]:     #18 0x40dcf3 in main ../src/userdb/userdbctl.c:1330
[   49.280657] testsuite-46.sh[1862]:     #19 0x7f4db9e4a50f in __libc_start_call_main (/lib64/libc.so.6+0x2750f)
[   49.280907] testsuite-46.sh[1862]: SUMMARY: AddressSanitizer: 103 byte(s) leaked in 14 allocation(s).