cifsd: Fix potential null-ptr-deref in destroy_previous_session()

cifsd: Fix potential null-ptr-deref in destroy_previous_session()

The user field in the session structure is allocated when the client is
authenticated. If the client explicitly logs off, the user field is freed,
but the session is kept around in case the user reconnects. If the TCP
connection hasn't been closed and the client sends a session setup with
a PreviousSessionId set, destroy_previous_session() will be called to
check if the session needs to be cleaned up.

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>