Basic error checking for mach-o

Basic error checking for mach-o

Fixes lots of places the fuzzers are going to find, and the one they
already hit.

	* mach-o.c (bfd_mach_o_read_dylinker): Don't read past end of
	command.  Check name offset is within command.
	(bfd_mach_o_read_dylib, bfd_mach_o_read_prebound_dylib),
	(bfd_mach_o_read_prebind_cksum, bfd_mach_o_read_twolevel_hints),
	(bfd_mach_o_read_fvmlib, bfd_mach_o_read_dysymtab),
	(bfd_mach_o_read_symtab, bfd_mach_o_read_uuid),
	(bfd_mach_o_read_linkedit, bfd_mach_o_read_str),
	(bfd_mach_o_read_dyld_info, bfd_mach_o_read_version_min),
	(bfd_mach_o_read_encryption_info, bfd_mach_o_read_source_version),
	(bfd_mach_o_read_encryption_info_64, bfd_mach_o_read_main),
	(bfd_mach_o_read_note, bfd_mach_o_read_build_version),
	(bfd_mach_o_read_segment): Similarly.
	(bfd_mach_o_read_thread): Properly bound check thread struct.
	Don't repeat checks on second loop.
	(bfd_mach_o_read_command): Fail on invalid command length.