wxString iterators can't be dereferenced once they reach the end of the
string, so compare them with end rather than checking if the value they point
to is non-NUL.
This makes wxFTP::Pwd() actually work, which was apparently broken since quite
some time (perhaps ever since c9f7896861 9+
years ago).
Closes#17679.
Since minimal.vcxproj file is shared by VS 2010-15 solution files, ToolsVersion value should be set to the value supported by all VS versions since 2010 to avoid warning messages like "Project file contains ToolsVersion="14.0", which is not supported by this version of MSBuild.".
It's impractical to have manual projects for all the samples, but do it at
least for this one to serve as a starting point for people using these IDEs.
