kernel: KVM: arm64: vgic-v2: Check for non-NULL vCPU in vgic_v2_parse_attr()

In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: vgic-v2: Check for non-NULL vCPU in vgicv2parseattr vgicv2parseattr is responsible for finding the vCPU that matches the user-provided CPUID, which of course may not be valid. If the ID is invalid, kvmgetvcpubyid...