Bug in Relation::members_str

[sulakm]

Hi, there is a bug in method Relation::members_str.

If m_WayRefs is empty, string is written at negative index. This bug causes crash after adding restriction tag for relation element into configuration xml.

Minimalist fix would be adding space into std::string way_list("");

More efficient fix is replacing way_list[way_list.size() -1] = ' '; with this:

	size_t n = way_list.size();
	if (n > 0) way_list.resize(n - 1);

Cheers, Martin