KVM: nVMX: Suppress external interrupt VM-Exit injection if there's no IRQ

KVM: nVMX: Suppress external interrupt VM-Exit injection if there's no IRQ

In the should-be-impossible scenario that kvm_cpu_get_interrupt() doesn't
return a valid vector after checking kvm_cpu_has_interrupt(), skip VM-Exit
injection to reduce the probability of crashing/confusing L1.  Now that
KVM gets the IRQ _before_ calling nested_vmx_vmexit(), squashing the
VM-Exit injection is trivial since there are no actions that need to be
undone.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20240906043413.1049633-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>