CFile: fix null pointer dereference in Stat() and Write()
[vfs] CFile: add null pointers checks
adds left, top, right, bottom, centerx, centery to the list of xml nodes that can take constants
support positioning dialogs via <left> and <top>
ensure library:// folder URLs have slash at end
Merge pull request #3534 from Karlson2k/cfile_fix_01

CFile: fix null pointer dereference