firewall3: Fix GCC8 warnings by replacing sprintf with snprintf

firewall3: Fix GCC8 warnings by replacing sprintf with snprintf

error: ‘%u’ directive writing between 1 and 10 bytes into a region of size
between 7 and 11 [-Werror=format-overflow=]
sprintf(buf, "%u-%u", port->port_min, port->port_max);
                 ^~
note: directive argument in the range [0, 2147483647]
sprintf(buf, "%u-%u", port->port_min, port->port_max);
             ^~~~~~~
note: ‘sprintf’ output between 4 and 17 bytes into a destination of size
13
sprintf(buf, "%u-%u", port->port_min, port->port_max);

Signed-off-by: Rosen Penev <rosenp@gmail.com>