panic: Decrementing non-positive ref count

goroutine 1848276 [running]:
gvisor.googlesource.com/gvisor/pkg/refs.(*AtomicRefCount).DecRefWithDestructor(0xc420fa0540, 0xc421525d58)
	pkg/refs/refcounter.go:271 +0x162
gvisor.googlesource.com/gvisor/pkg/sentry/kernel/shm.(*Shm).DecRef(0xc420fa0540)
	pkg/sentry/kernel/shm/shm.go:373 +0x48
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).removeVMAsLocked(0xc422d6cc80, 0xc82100, 0xc420a29400, 0x1000, 0x7ffffffff000, 0x67c91d, 0xc421525e48)
	pkg/sentry/mm/vma.go:338 +0x1a4
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).unmapLocked(0xc422d6cc80, 0xc82100, 0xc420a29400, 0x1000, 0x7ffffffff000, 0x18, 0xc421b029b0)
	pkg/sentry/mm/vma.go:310 +0x7e
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).DecUsers(0xc422d6cc80, 0xc82100, 0xc420a29400)
	pkg/sentry/mm/lifecycle.go:220 +0x1dd
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*TaskContext).release(0xc4207cbd30)
	pkg/sentry/kernel/task_context.go:62 +0xd6
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*runExitMain).execute(0x0, 0xc4207cb600, 0xc74ca0, 0x0)
	pkg/sentry/kernel/task_exit.go:261 +0xc5
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).run(0xc4207cb600, 0x452c)
	pkg/sentry/kernel/task_run.go:95 +0x174
created by gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).Start
	pkg/sentry/kernel/task_start.go:258 +0x100