[mosquitto-dev] mosquitopp and usage of std::string

[Kiran Pradeep <kiran.happy@xxxxxxxxx>]

Is the usage of `const char*` over `std::string` in `mosquittopp.h`
intentional ? If not, shall I try a simple pull request on develop
branch ? I intent to replace char*/int* with std::string/int& with
assistance of a helper function like

const char* empty_to_null(const std::string& str)
{
	return (str.empty() == true) ? NULL : str.c_str();
}

Thanks,
Kiran.