RISC-V: Removed the redundant NULL pointer check in the riscv_update_subset.

If we always use the .option arch to call the riscv_update_subset, then
it is almost impossible that the input string will be NULL.  Therefore,
just remove the redundant NULL pointer check in the riscv_update_subset.

bfd/
	* elfxx-riscv.c (riscv_update_subset): Removed the redundant NULL
	pointer check.