Fix a bad free on portfwd name resolution failure.

If name resolution fails, pfd_connect tries to sfree(dummy_realhost)
when it's completely uninitialised - the failed resolution didn't
write to it, and we also didn't precautionarily initialise it to NULL
first. Now we do the latter.

portfwd.c

@@ -433,7 +433,7 @@ char *pfd_connect(struct PortForwarding **pf_ret, char *hostname,int port,
 
     SockAddr addr;
     const char *err;
-    char *dummy_realhost;
+    char *dummy_realhost = NULL;
     struct PortForwarding *pf;
 
     /*