socket: don't attempt to reconnect a TCP socket in server mode

Commit c3767ed0eb introduced a possible SEGV when
using a socket chardev with server=on because it assumes that all TCP sockets
are in client mode.

This patch adds a check to only reconnect when in client mode.

Cc: Lei Li <lilei@linux.vnet.ibm.com>
Reported-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-char.c

@@ -2148,10 +2148,12 @@ static int tcp_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
     TCPCharDriver *s = chr->opaque;
     if (s->connected) {
         return send_all(s->fd, buf, len);
-    } else {
+    } else if (s->listen_fd == -1) {
         /* (Re-)connect for unconnected writing */
         tcp_chr_connect(chr);
         return 0;
+    } else {
+        return len;
     }
 }