RISC-V: KVM: Fix NULL pointer dereference in AIA IMSIC functions

Fuzzer reported a NULL pointer dereference in
kvm_riscv_vcpu_aia_imsic_put() when a VCPU's imsic_state was NULL while
kvm_riscv_aia_initialized() returned true.

The global initialized flag is set per-VM in aia_init(), but imsic_state
is allocated per-VCPU in kvm_riscv_vcpu_aia_imsic_init(). If a VCPU is
created after aia_init() has already run, its imsic_state remains NULL
while the global flag is true. When this VCPU is preempted, kvm_sched_out()
calls kvm_arch_vcpu_put() -> kvm_riscv_vcpu_aia_put() ->
kvm_riscv_vcpu_aia_imsic_put() which dereferences NULL.

Add NULL pointer guards to kvm_riscv_vcpu_aia_imsic_put(), consistent with
the NULL checks already present in all other functions in the same file.

Also add a NULL guard to kvm_riscv_vcpu_aia_imsic_release() and
kvm_riscv_vcpu_aia_imsic_has_interrupt() for the same reason.

Fixes: 4cec89db80ba ("RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization")
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Assisted-by: YuanSheng:DeepSeek-V3.2
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260526031517.1166025-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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