target/riscv: Fix null pointer dereference in cpu_set_exception_base

Add missing return statement after logging the error when cs is NULL.
Without this, the function continues to dereference the null pointer.

Resolves: Coverity CID 1644077

Signed-off-by: Djordje Todorovic <djordje.todorovic@htecgroup.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260115130110.2825796-3-djordje.todorovic@htecgroup.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>