config: fix valgrind leak

Running 'criu dump -t <PID>' with a configuration file under valgrind
where <PID> does not exist, gives:

==14336== 600 bytes in 5 blocks are definitely lost in loss record 5 of 5
==14336==    at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==14336==    by 0x5D387A4: getdelim (in /usr/lib64/libc-2.17.so)
==14336==    by 0x439829: getline (stdio.h:117)
==14336==    by 0x439829: parse_config (config.c:69)
==14336==    by 0x439CB2: init_configuration.isra.1 (config.c:159)
==14336==    by 0x439F75: init_config (config.c:212)
==14336==    by 0x439F75: parse_options (config.c:487)
==14336==    by 0x42499F: main (crtools.c:140)

==14336== LEAK SUMMARY:
==14336==    definitely lost: 600 bytes in 5 blocks

With this patch:

==17892== LEAK SUMMARY:
==17892==    definitely lost: 0 bytes in 0 blocks

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>