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

Add a null pointer check for imsic_state before dereferencing it in
kvm_riscv_aia_imsic_has_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.

This issue was discovered during fuzzing of RISC-V KVM code. The
crash occurs when userspace calls KVM_HAS_DEVICE_ATTR ioctl on an
AIA IMSIC device before the IMSIC state has been fully initialized
for a vcpu.

The crash manifests as:
Unable to handle kernel paging request at virtual address
dfffffff00000001
...
epc : kvm_riscv_aia_imsic_has_attr+0x464/0x50e
arch/riscv/kvm/aia_imsic.c:998
...
kvm_riscv_aia_imsic_has_attr+0x464/0x50e arch/riscv/kvm/aia_imsic.c:998
aia_has_attr+0x128/0x2bc arch/riscv/kvm/aia_device.c:471
kvm_device_ioctl_attr virt/kvm/kvm_main.c:4722 [inline]
kvm_device_ioctl+0x296/0x374 virt/kvm/kvm_main.c:4739
...

The fix adds a check to return -ENODEV if imsic_state is NULL, which
is consistent with other error handling in the function and prevents
the null pointer dereference.

Fixes: 5463091a51cf ("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: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260125143344.2515451-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>