cache_lock: do xstrdup/free on lockfile

cache_lock: do xstrdup/free on lockfile

Since fmt() uses 8 alternating static buffers, and cache_lock might call
cache_create_dirs() multiple times, which in turn might call fmt() twice,
after four iterations lockfile would be overwritten by a cachedirectory
path.

In worst case, this could cause the cachedirectory to be unlinked and replaced
by a cachefile.

Fix: use xstrdup() on the result from fmt() before assigning to lockfile, and
call free(lockfile) before exit.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>