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>

target/riscv/cpu.c

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index e56470a..9be7962 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -83,6 +83,7 @@
         qemu_log_mask(LOG_GUEST_ERROR,
                       "cpu_set_exception_base: invalid vp_index: %u",
                       vp_index);
+        return;
     }
     cpu = RISCV_CPU(cs);
     cpu->env.resetvec = address;