libmount: free username after check

libmount: free username after check

Leak reported by valgrind:

==14226== 7 bytes in 1 blocks are definitely lost in loss record 1 of 1
==14226==    at 0x4C2757B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14226==    by 0x5534839: strdup (in /usr/lib/libc-2.18.so)
==14226==    by 0x4E53FE0: mnt_get_username (utils.c:560)
==14226==    by 0x4E456A5: mnt_context_prepare_umount (context_umount.c:413)
==14226==    by 0x4E464F7: mnt_context_umount (context_umount.c:851)
==14226==    by 0x403476: umount_one (umount.c:299)
==14226==    by 0x402B34: main (umount.c:629)

Signed-off-by: Dave Reisner <dreisner@archlinux.org>