libblkid: fix blkid_parse_tag_string() to ignore incomplete tags

fstab entry:

    FOO=     /       btrfs  defaults       1 1

Program received signal SIGABRT, Aborted.
0x00007f016ccd8819 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56   return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
(gdb) bt
    at ../sysdeps/unix/sysv/linux/libc_fatal.c:196

Reported-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Karel Zak <kzak@redhat.com>