Fix a potential crash in ssh_setup_portfwd.

If we search for a colon by computing ptr + host_strcspn(ptr,":"),
then the resulting pointer is always non-NULL, and the 'not found'
condition is not !p but !*p.

This typo could have caused PuTTY to overrun a string, but not in a
security-bug sense because any such string would have to have been
loaded from the configuration rather than received from a hostile
source.

[originally from svn r10123]

ssh.c

@@ -4955,7 +4955,7 @@ static void ssh_setup_portfwd(Ssh ssh, Conf *conf)
 	    vp = val;
 	    vp2 = vp + host_strcspn(vp, ":");
 	    host = dupprintf("%.*s", (int)(vp2 - vp), vp);
-	    if (vp2)
+	    if (*vp2)
 		vp2++;
 	    dports = vp2;
 	    dport = atoi(dports);