RISC-V: KVM: Skip IMSIC update if vCPU IMSIC state is not initialized

kvm_riscv_vcpu_aia_imsic_update() assumes that the vCPU IMSIC state has
already been initialized and unconditionally accesses imsic->vsfile_lock.
However, in fuzzed ioctl sequences, the AIA device may be initialized at
the VM level while the per-vCPU IMSIC state is still NULL.

This leads to invalid access when entering the vCPU run loop before
IMSIC initialization has completed.

The crash manifests as:
Unable to handle kernel paging request at virtual address
dfffffff00000006
...
kvm_riscv_vcpu_aia_imsic_update arch/riscv/kvm/aia_imsic.c:801
kvm_riscv_vcpu_aia_update arch/riscv/kvm/aia_device.c:493
kvm_arch_vcpu_ioctl_run arch/riscv/kvm/vcpu.c:927
...

Add a guard to skip the IMSIC update path when imsic_state is NULL. This
allows the vCPU run loop to continue safely.

This issue was discovered during fuzzing of RISC-V KVM code.

Fixes: db8b7e97d6137a ("RISC-V: KVM: Add in-kernel virtualization of AIA IMSIC")
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/20260127084313.3496485-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>