mkswap: remove memory leaks [LeakSanitizer] [valgrind]

mkswap: remove memory leaks [LeakSanitizer] [valgrind]

==18922==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x49d12b in __interceptor_malloc (/home/src/util-linux/.libs/lt-mkswap+0x49d12b)
    #1 0x7faf2a5069c9 in __GI___strdup (/usr/lib/libc.so.6+0x819c9)
    #2 0xffff96e7e33 (<unknown module>)

SUMMARY: AddressSanitizer: 8 byte(s) leaked in 1 allocation(s).

And another one that valgrind found.

==6316== 8 bytes in 1 blocks are definitely lost in loss record 1 of 1
==6316==    at 0x4C29F90: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6316==    by 0x5E3F9C9: strdup (in /usr/lib/libc-2.20.so)
==6316==    by 0x43A25F: size_to_human_string (strutils.c:495)
==6316==    by 0x42B35C: main (mkswap.c:488)

Signed-off-by: Sami Kerola <kerolasa@iki.fi>