RISC-V: KVM: Fix null pointer dereference in kvm_riscv_aia_imsic_rw_attr()

Add a null pointer check for imsic_state before dereferencing it in
kvm_riscv_aia_imsic_rw_attr(). While the function checks that the
vcpu exists, it doesn't verify that the vcpu's imsic_state has been
initialized, leading to a null pointer dereference when accessed.

The crash manifests as:
Unable to handle kernel paging request at virtual address
dfffffff00000006
...
kvm_riscv_aia_imsic_rw_attr+0x2d8/0x854 arch/riscv/kvm/aia_imsic.c:958
aia_set_attr+0x2ee/0x1726 arch/riscv/kvm/aia_device.c:354
kvm_device_ioctl_attr virt/kvm/kvm_main.c:4744 [inline]
kvm_device_ioctl+0x296/0x374 virt/kvm/kvm_main.c:4761
vfs_ioctl fs/ioctl.c:51 [inline]
...

The fix adds a check to return -ENODEV if imsic_state is NULL and moves
isel assignment after imsic_state NULL check.

Fixes: 5463091a51cfaa ("RISC-V: KVM: Expose IMSIC registers as attributes of AIA irqchip")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260127072219.3366607-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>