Fix a bug occuring when using certain actions on a collection

Fix a bug occuring when using certain actions on a collection
item which is not in the selection.
Triggering it will either result in an assertion failure or a
segfault.

To reproduce:
- open a collection
- right-click on a non-selected image
- select Delete... menu entry (or Copy, Move, Rename, Properties...)

It results in the best case:
** ERROR **: file filelist.c: line 905 (file_data_ref): assertion failed: (fd->magick == 0x12345678)
or a pure segfault due to corrupted data.

collection_table_popup_file_list() has to return a list of filedata *
not a list of gchar *.