RISC-V: KVM: Fix NULL pointer dereference in SBI v0.1 SEND_IPI handler

The SBI v0.1 SEND_IPI handler iterates over the hart mask and calls
kvm_get_vcpu_by_id() to find the target vcpu for each set bit. When a
guest provides a hart mask containing bits for non-existent vcpu_ids,
kvm_get_vcpu_by_id() returns NULL, which is then unconditionally
dereferenced by kvm_riscv_vcpu_set_interrupt(), causing a kernel crash.

Fix this by adding a NULL check before dereferencing the return value.
If the target vcpu is not found, skip it and continue processing the
remaining valid harts.

Fixes: a046c2d ("RISC-V: KVM: Reorganize SBI code by moving SBI v0.1 to its own file")
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Assisted-by: OpenClaw:DeepSeek-V3.2
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260517124414.420919-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: 6eanut

arch/riscv/kvm/vcpu_sbi_v01.c

@@ -55,6 +55,8 @@ static int kvm_sbi_ext_v01_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
 
 		for_each_set_bit(i, &hmask, BITS_PER_LONG) {
 			rvcpu = kvm_get_vcpu_by_id(vcpu->kvm, i);
+			if (!rvcpu)
+				continue;
 			ret = kvm_riscv_vcpu_set_interrupt(rvcpu, IRQ_VS_SOFT);
 			if (ret < 0)
 				break;