sysusers: fix memleak (#4430)

Fixes:
```
==28075== 64 bytes in 1 blocks are definitely lost in loss record 2 of 3
==28075==    at 0x4C2BAEE: malloc (vg_replace_malloc.c:298)
==28075==    by 0x4C2DCA1: realloc (vg_replace_malloc.c:785)
==28075==    by 0x4ED40A2: greedy_realloc (alloc-util.c:57)
==28075==    by 0x4E90F87: extract_first_word (extract-word.c:78)
==28075==    by 0x4E91813: extract_many_words (extract-word.c:270)
==28075==    by 0x10FE93: parse_line (sysusers.c:1325)
==28075==    by 0x11198B: read_config_file (sysusers.c:1640)
==28075==    by 0x111EB8: main (sysusers.c:1773)
==28075==
```