libmount: fix memory overflow [AddressSanitizer]

libmount: fix memory overflow [AddressSanitizer]

==10918==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fffd795b680 at pc 0x0000004447c6 bp 0x7fffd795b3e0 sp 0x7fffd795ab78
WRITE of size 129 at 0x7fffd795b680 thread T0
    #0 0x4447c5 in scanf_common(void*, int, bool, char const*, __va_list_tag*) (/home/src/util-linux/.libs/lt-mount+0x4447c5)
    #1 0x445892 in sscanf (/home/src/util-linux/.libs/lt-mount+0x445892)
    #2 0x7fe78709a3d3 in get_filesystems /home/src/util-linux/libmount/src/utils.c:581:7
    #3 0x7fe78709a1ba in mnt_get_filesystems /home/src/util-linux/libmount/src/utils.c:622:7
    #4 0x7fe7870aa78f in do_mount_by_pattern /home/src/util-linux/libmount/src/context_mount.c:833:7
    #5 0x7fe7870a9534 in mnt_context_do_mount /home/src/util-linux/libmount/src/context_mount.c:951:9
    #6 0x7fe7870aab2b in mnt_context_mount /home/src/util-linux/libmount/src/context_mount.c:1051:8
    #7 0x4ba9f5 in main /home/src/util-linux/sys-utils/mount.c:1107:7
    #8 0x7fe785caa03f in __libc_start_main (/usr/lib/libc.so.6+0x2003f)
    #9 0x4b9f9c in _start (/home/src/util-linux/.libs/lt-mount+0x4b9f9c)

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>