Fix a seg-fault when displaying notes from a prorgam segment. Check for a NULL strin...

Fix a seg-fault when displaying notes from a prorgam segment.  Check for a NULL string section before attempting compute the name of a separate debug info file.

	PR 22490
	* readelf.c (dump_section_as_bytes): Check for a NULL string
	section pointer.
	(process_notes_at): Compute the alignment to use if displaying
	notes from a segment.