tcp_ctl: use slirp_fmt()

Make it safer to OOB (sb_cc must not go out of sb_data), warn on
truncation, abort on error.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20200127092414.169796-6-marcandre.lureau@redhat.com>

src/tcp_subr.c

@@ -965,9 +965,8 @@ int tcp_ctl(struct socket *so)
             }
         }
     }
-    sb->sb_cc =
-        snprintf(sb->sb_wptr, sb->sb_datalen - (sb->sb_wptr - sb->sb_data),
-                 "Error: No application configured.\r\n");
+    sb->sb_cc = slirp_fmt(sb->sb_wptr, sb->sb_datalen - (sb->sb_wptr - sb->sb_data),
+                          "Error: No application configured.\r\n");
     sb->sb_wptr += sb->sb_cc;
     return 0;
 }