_ostree_kernel_args_replace_take: don't leak when replacing

_ostree_kernel_args_replace_take: don't leak when replacing

If !existed, then we add arg to kargs->order, where it will be freed
by that array's free-function. However, if the kernel argument did
already exist, we have to either free arg ourselves (and make sure
the old key is what appears in the hash table), or do a linear search
on kargs->order to replace the old key with the new.

Leak found by valgrind memcheck.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #559
Approved by: cgwalters