RISC-V: PR28610, Fix ASAN heap-buffer-overflow error in riscv_update_subset.

The architecture parser in riscv_update_subset shouldn't check (or access)
the pointer space which doesn't exist.

bfd/
	pr 28610
	* elfxx-riscv.c (riscv_update_subset): The architecture parser
	shouldn't access the pointer space which doesn't exist.