DWARF header checks

DWARF header checks

This patch tidies DWARF header checks, consolidating the "negative"
checks (which are really overflow checks) with the section size
check.  In a number of cases this also ensures that small negative
lengths are caught.  For instance

      hdrptr = start + arange.ar_length + initial_length_size;
      if (hdrptr < start || hdrptr > end)

does not detect ar_length in the range [-initial_length_size,-1].

	* dwarf.c (process_debug_info): Consolidate header length checks.
	(display_debug_pubnames_worker): Use "start" to read header.
	Properly check header length and report errors earlier.
	Simplify loop printing pubnames.
	(get_line_filename_and_dirname): Catch small negative "length"
	values.
	(display_debug_aranges): Likewise.  Report header errors
	earlier using standardized message.
	(display_debug_names): Likewise.