Fix string iteration logic in wxFTP::Pwd()

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.

samples/minimal/minimal.vcxproj

@@ -422,4 +422,4 @@
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
-</Project>
+</Project>