msan: bfd_mach_o_layout_commands use of uninitialised value

Catches fuzzed input with unterminated strings that later run off the
end of their buffers when calling strlen.

	* mach-o.c: Use size_t vars where approprite.
	(bfd_mach_o_alloc_and_read): Add "extra" param.  Allocate that
	much extra and clear.  Update all callers, those that set up
	strings with one extra byte.