lstab: Don't free lstab on update.

The modification timestamp of the leapfile is checked with every
call of lstab_utc2tai(). If the file is modified, the provided lstab
structure is freed and a new one is allocated from the updated leapfile.
But the new lstab is not returned to the caller as the function doesn't
accept a pointer to the pointer to lstab. This causes reading from the
freed memory and leak of the newly allocated memory.

Modify update_leapsecond_table() to read the updated leapfile into the
existing lstab structure instead of the reallocation.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>