Set piece size to (max) 4Mb instead of empty string when created torrent
is very large.
Fix emptying of piece size combobox when user checks "auto" checkbox.

The function computePathSize returns -1 if file isn't found, not 0.
Merge pull request #2174 from sorokin/fix-empty-combobox

Fix emptying of 'piece size' combobox when user enable 'auto' checkbox and no files are selected