macho null dereference read

The main problem here was not returning -1 from canonicalize_symtab on
an error, leaving the vector of relocs only partly initialised and one
with a null sym_ptr_ptr.

	* mach-o.c (bfd_mach_o_canonicalize_symtab): Return -1 on error,
	not 0.
	(bfd_mach_o_pre_canonicalize_one_reloc): Init sym_ptr_ptr to
	undefined section sym.