Bug 7000028 - Drag-n-drop export of a file produces 0 sized PDF file

em_utils_print_messages_to_file() was doing so asynchronously, but
unfortunately drag-n-drop is a synchronous protocol.  So by the time
the asynchronous print operation completed, the URI list pointing to
the temporary PDF files had already been passed to the file manager.

The only reason the files were created at all was because we test the
generated file name with open(...O_CREAT...) before starting the print
operation, and I'm not convinced that test is even necessary.

(cherry picked from commit 72f465b1)