sysusers: fix memleak (#4443)

Fixes:
Oct 20 09:10:49 systemd-sysusers[144]: Direct leak of 20 byte(s) in 1 object(s) allocated from:
Oct 20 09:10:49 systemd-sysusers[144]:     #0 0x7f3565a13e60 in malloc (/lib64/libasan.so.3+0xc6e60)
Oct 20 09:10:49 systemd-sysusers[144]:     #1 0x7f3565526bd0 in malloc_multiply src/basic/alloc-util.h:70
Oct 20 09:10:49 systemd-sysusers[144]:     #2 0x7f356552cb55 in tempfn_xxxxxx src/basic/fileio.c:1116
Oct 20 09:10:49 systemd-sysusers[144]:     #3 0x7f356552c4f0 in fopen_temporary src/basic/fileio.c:1042
Oct 20 09:10:49 systemd-sysusers[144]:     #4 0x7f356555e00e in fopen_temporary_label src/basic/fileio-label.c:63
Oct 20 09:10:49 systemd-sysusers[144]:     #5 0x56197c4a1766 in make_backup src/sysusers/sysusers.c:209
Oct 20 09:10:49 systemd-sysusers[144]:     #6 0x56197c4a6335 in write_files src/sysusers/sysusers.c:710
Oct 20 09:10:49 systemd-sysusers[144]:     #7 0x56197c4ae571 in main src/sysusers/sysusers.c:1817
Oct 20 09:10:49 systemd-sysusers[144]:     #8 0x7f3564dee730 in __libc_start_main (/lib64/libc.so.6+0x20730)

(cherry picked from commit 0a12bb1e)