[CIFS] Possible null ptr deref in SMB2_tcon

As Raphael Geissert pointed out, tcon_error_exit can dereference tcon
and there is one path in which tcon can be null.

Signed-off-by: Steve French <smfrench@gmail.com>
CC: Stable <stable@vger.kernel.org> # v3.7+
Reported-by: Raphael Geissert <geissert@debian.org>

fs/cifs/smb2pdu.c

@@ -907,7 +907,8 @@ tcon_exit:
 tcon_error_exit:
 	if (rsp->hdr.Status == STATUS_BAD_NETWORK_NAME) {
 		cifs_dbg(VFS, "BAD_NETWORK_NAME: %s\n", tree);
-		tcon->bad_network_name = true;
+		if (tcon)
+			tcon->bad_network_name = true;
 	}
 	goto tcon_exit;
 }