dwarf2.c: read_abbrevs fail cleanup, and offset checking

dwarf2.c: read_abbrevs fail cleanup, and offset checking

read_section does offset checking, reporting an error on out of
bounds.  There's no need to duplicate the check in functions calling
read_section.  Also, I spotted a place where a pointer difference
expression was being cast to unsigned int, possibly truncating
relevant bits on a 64-bit host.

	* dwarf2.c (read_indirect_string): Don't duplicate offset check
	done in read_section.
	(read_indirect_line_string): Likewise.
	(read_alt_indirect_string): Likewise.
	(read_alt_indirect_ref): Likewise.
	(read_abbrevs): Likewise.  Free memory on all failure paths.
	Use correct unsigned type for pointer difference comparison.