[CIFS] Do not send tree disconnect if session is already disconnected

Noticed this when tree connect timed out (due to Samba server crash) -
we try to send a tree disconnect for a tid that does not exist
since we don't have a valid tree id yet. This checks that the
session is valid before sending the tree disconnect to handle
this case.

Signed-off-by: Steve French <sfrench@us.ibm.com>