Fix an indirection via uninitialised memory when parsing a corrupt input file.

	PR 27839
	* simple.c (simple_dummy_add_to_set): New function.
	(simple_dummy_constructor): New function.
	(simple_dummy_multiple_common): New function.
	(bfd_simple_get_relocated_section_contents): Zero out the
	callbacks structure, and then set the add_to_set, constructor and
	multiple_common fields.