smb3: add additional null check in SMB2_tcon

Although unlikely to be possible for rsp to be null here,
the check is safer to add, and quiets a Coverity warning.

Addresses-Coverity: 1420428 ("Explicit null dereferenced")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -1840,7 +1840,7 @@ SMB2_tcon(const unsigned int xid, struct cifs_ses *ses, const char *tree,
 	cifs_small_buf_release(req);
 	rsp = (struct smb2_tree_connect_rsp *)rsp_iov.iov_base;
 	trace_smb3_tcon(xid, tcon->tid, ses->Suid, tree, rc);
-	if (rc != 0) {
+	if ((rc != 0) || (rsp == NULL)) {
 		cifs_stats_fail_inc(tcon, SMB2_TREE_CONNECT_HE);
 		tcon->need_reconnect = true;
 		goto tcon_error_exit;