Fix VD-1301 and VD-1302 vulnerabilities

This patch was contributed by Maor Vermucht and Or Peles from
VDOO Connected Trust.
void modbus_free(modbus_t *ctx)
{
    if (ctx == NULL)
        return;

    ctx->backend->free(ctx);
}

void modbus_free(modbus_t *ctx)
{
    if (ctx == NULL)
        return;

    ctx->backend->free(ctx);
}
