Don't immediately segfault with -rtos on v0.11.

Author: timsifive

src/target/riscv/riscv.c

@@ -1012,6 +1012,9 @@ bool riscv_rtos_enabled(const struct target *target)
 void riscv_set_current_hartid(struct target *target, int hartid)
 {
 	RISCV_INFO(r);
+	if (!r->select_current_hart)
+		return;
+
 	int previous_hartid = riscv_current_hartid(target);
 	r->current_hartid = hartid;
 	assert(riscv_rtos_enabled(target) || target->coreid == hartid);