slirp: Fix error reported by static code analysis

Report from smatch:

slirp/tcp_subr.c:127 tcp_respond(17) error:
 we previously assumed 'tp' could be null (see line 124)

Return if 'tp' is NULL.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
(cherry picked from commit e56afbc)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

slirp/tcp_subr.c

@@ -124,7 +124,7 @@ tcp_respond(struct tcpcb *tp, struct tcpiphdr *ti, struct mbuf *m,
 	if (tp)
 		win = sbspace(&tp->t_socket->so_rcv);
         if (m == NULL) {
-		if ((m = m_get(tp->t_socket->slirp)) == NULL)
+		if (!tp || (m = m_get(tp->t_socket->slirp)) == NULL)
 			return;
 		tlen = 0;
 		m->m_data += IF_MAXLINKHDR;