Prevent a double free by not reentering be_tls_close().

Reentering this function with the right timing caused a double free,
typically crashing the backend.  By synchronizing a disconnection with
the authentication timeout, an unauthenticated attacker could achieve
this somewhat consistently.  Call be_tls_close() solely from within
proc_exit_prepare().  Back-patch to 9.0 (all supported versions).

Benkocs Norbert Attila

Security: CVE-2015-3165