PR29872, uninitialised value in display_debug_lines_decoded dwarf.c:5413

Plus segvs if the C-library doesn't handle printf %s of NULL.

	PR 29872
	* dwarf.c (null_name): New function.
	(process_debug_info): Use it here..
	(display_debug_lines_raw): ..and here..
	(display_debug_lines_decoded): ..and here.  xcalloc directory_table.
	Simplify xcalloc of file_table.