Fix another leak

==13267== 624 (144 direct, 480 indirect) bytes in 6 blocks are definitely lost in loss record 14,700 of 15,489
==13267== at 0x4C2779D: malloc (vg_replace_malloc.c:236)
==13267== by 0x9014392: g_malloc (gmem.c:164)
==13267== by 0x902AFE6: g_slice_alloc (gslice.c:842)
==13267== by 0x90084BD: g_list_prepend (glist.c:309)
==13267== by 0x932797F: purple_log_get_log_sets (log.c:1087)
==13267== by 0x4B3929: add_completion_list (gtkutils.c:2093)