silence compiler warnings for mingw win32 builds --enable-debug

lib/ftp.c

@@ -1062,7 +1062,7 @@ static CURLcode ftp_state_use_port(struct connectdata *conn,
       /* we set the secondary socket variable to this for now, it
          is only so that the cleanup function will close it in case
          we fail before the true secondary stuff is made */
-      if(-1 != conn->sock[SECONDARYSOCKET])
+      if(CURL_SOCKET_BAD != conn->sock[SECONDARYSOCKET])
         sclose(conn->sock[SECONDARYSOCKET]);
       conn->sock[SECONDARYSOCKET] = portsock;