maintenance: fix two memory leaks

maintenance: fix two memory leaks

Fixes two memory leaks when running `git maintenance start` or `git
maintenance stop` in `update_background_schedule`:

$ valgrind --leak-check=full ~/git/bin/git maintenance start
==76584== Memcheck, a memory error detector
==76584== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==76584== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==76584== Command: /home/lenaic/git/bin/git maintenance start
==76584==
==76584==
==76584== HEAP SUMMARY:
==76584==     in use at exit: 34,880 bytes in 252 blocks
==76584==   total heap usage: 820 allocs, 568 frees, 146,414 bytes allocated
==76584==
==76584== 65 bytes in 1 blocks are definitely lost in loss record 17 of 39
==76584==    at 0x483E6AF: malloc (vg_replace_malloc.c:306)
==76584==    by 0x3DC39C: xrealloc (wrapper.c:126)
==76584==    by 0x3992CC: strbuf_grow (strbuf.c:98)
==76584==    by 0x39A473: strbuf_vaddf (strbuf.c:392)
==76584==    by 0x39BC54: xstrvfmt (strbuf.c:979)
==76584==    by 0x39BD2C: xstrfmt (strbuf.c:989)
==76584==    by 0x18451B: update_background_schedule (gc.c:1977)
==76584==    by 0x1846F6: maintenance_start (gc.c:2011)
==76584==    by 0x1847B4: cmd_maintenance (gc.c:2030)
==76584==    by 0x127A2E: run_builtin (git.c:453)
==76584==    by 0x127E81: handle_builtin (git.c:704)
==76584==    by 0x128142: run_argv (git.c:771)
==76584==
==76584== 240 bytes in 1 blocks are definitely lost in loss record 29 of 39
==76584==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==76584==    by 0x491CE5D: getdelim (in /usr/lib/libc-2.33.so)
==76584==    by 0x39ADD7: strbuf_getwholeline (strbuf.c:635)
==76584==    by 0x39AF31: strbuf_getdelim (strbuf.c:706)
==76584==    by 0x39B064: strbuf_getline_lf (strbuf.c:727)
==76584==    by 0x184273: crontab_update_schedule (gc.c:1919)
==76584==    by 0x184678: update_background_schedule (gc.c:1997)
==76584==    by 0x1846F6: maintenance_start (gc.c:2011)
==76584==    by 0x1847B4: cmd_maintenance (gc.c:2030)
==76584==    by 0x127A2E: run_builtin (git.c:453)
==76584==    by 0x127E81: handle_builtin (git.c:704)
==76584==    by 0x128142: run_argv (git.c:771)
==76584==
==76584== LEAK SUMMARY:
==76584==    definitely lost: 305 bytes in 2 blocks
==76584==    indirectly lost: 0 bytes in 0 blocks
==76584==      possibly lost: 0 bytes in 0 blocks
==76584==    still reachable: 34,575 bytes in 250 blocks
==76584==         suppressed: 0 bytes in 0 blocks
==76584== Reachable blocks (those to which a pointer was found) are not shown.
==76584== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==76584==
==76584== For lists of detected and suppressed errors, rerun with: -s
==76584== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>