smb3: add additional null check in SMB2_open

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

Addresses-Coverity: 1418458 ("Explicit null dereferenced")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -2941,7 +2941,9 @@ SMB2_open(const unsigned int xid, struct cifs_open_parms *oparms, __le16 *path,
 			tcon->need_reconnect = true;
 		}
 		goto creat_exit;
-	} else
+	} else if (rsp == NULL) /* unlikely to happen, but safer to check */
+		goto creat_exit;
+	else
 		trace_smb3_open_done(xid, le64_to_cpu(rsp->PersistentFileId),
 				     tcon->tid,
 				     ses->Suid, oparms->create_options,