PR22205, .debug_line file table NULL filename

PR22205, .debug_line file table NULL filename

The PR22200 fuzzer testcase found one way to put NULLs into .debug_line
file tables.  PR22205 finds another.  This patch gives up on trying to
prevent NULL files in the file table and instead just copes with them.
Arguably, this is better than giving up and showing no info from
.debug_line.  I've also fixed a case where the fairly recent DWARF5
support in handling broken DWARG could result in uninitialized memory
reads, and made a small tidy.

	PR 22205
	* dwarf2.c (concat_filename): Return "<unknown>" on NULL filename.
	(read_formatted_entries): Init "fe".
	(decode_line_info <DW_LNE_define_file>): Use line_info_add_file_name.