child-sa: Update status flags based on success of SA addition

child-sa: Update status flags based on success of SA addition

Both variables `inbound_installed` and `outbound_state` are used in
`child_sa_t::destroy()` to determine whether inbound and outbound state
have to be deleted. They are assigned prior to the call to
`kernel_interface_t::add_sa()`. As this call may fail, the destructor may
try to delete a state which it has not been added.
By making the assignment of these variables dependent on the success of
the state addition, we can make sure, a `child_sa_t::destroy()` only
deletes states it has added.

Also removed the redundant checks for `my_spi` and `other_spi` being set
along with the check for the above flags. It seems that when the flags
are set, the SPIs *must* be set.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>