call: avoid to get access to invalidated account

getSIPAccount() is returning a reference to the SIPAccount. Account
that can be destroyed and if a mistake is made, the data can be
invalidated, generally even when closing the daemon.

This patch change the behavior of getSIPAccount() to return a
weak_ptr to make sure that the Account is alive when performing
actions on the call

Change-Id: I1dcc05bce0c9826297535b5a5224f9ff22479d80