Fix heap-buffer overflow bugs caused when dumping debug information from a corrupt...

Fix heap-buffer overflow bugs caused when dumping debug information from a corrupt binary.

	PR binutils/21438
	* dwarf.c (process_extended_line_op): Do not assume that the
	string extracted from the section is NUL terminated.
	(fetch_indirect_string): If the string retrieved from the section
	is not NUL terminated, return an error message.
	(fetch_indirect_line_string): Likewise.
	(fetch_indexed_string): Likewise.