sip: Fix increment of invite session

Increment of the invite session has to be done before SIPCall takes a reference
to it.

Otherwise, `pjsip_inv_dec_ref()` will be called internally by PJSIP when the
reference counter has reached 0.  This happen when `SIPCall::setInviteSession()`
is called in `invite_session_state_changed_cb()`, resulting in the deletion of
the invitation.

Change-Id: Icba0985ea26ce5dec462bd124cca4dd2187d6a47