riscv: kvm: add null pointer check for vector datap

riscv: kvm: add null pointer check for vector datap

Add WARN_ON check before accessing cntx->vector.datap in
kvm_riscv_vcpu_vreg_addr() to detect potential null pointer
dereferences early, consistent with the pattern used in
kvm_riscv_vcpu_vector_reset().

This helps catch initialization issues where vector context
allocation may have failed.

Signed-off-by: Yufeng Wang <wangyufeng@kylinos.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260317114759.53165-1-r4o5m6e8o@163.com
Signed-off-by: Anup Patel <anup@brainfault.org>