fst-manager: Fix potential use of unset variable and double free

In fst_disconnect_peer(), ret might be used uninitialized.
Fix this by setting ret to initial value.
In _fst_mgr_peer_set_active_iface(), addr might be accessed while
it is NULL.
Fix this by checking NULL value.
In fst_dup_connection_sta(), str_mbies might get freed twice.
Fix this by freeing str_mbies at the end of the function.

CRs-Fixed: 1097078

Change-Id: Ia3db5ee66fe775396faa74d82e20fbdad52d5077
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>