panic: Decrementing non-positive ref count

goroutine 4939977 [running]:
gvisor.googlesource.com/gvisor/pkg/refs.(*AtomicRefCount).DecRefWithDestructor(0xc423d566c0, 0xc421b1fd58)
	pkg/refs/refcounter.go:271 +0x162
gvisor.googlesource.com/gvisor/pkg/sentry/kernel/shm.(*Shm).DecRef(0xc423d566c0)
	pkg/sentry/kernel/shm/shm.go:359 +0x48
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).removeVMAsLocked(0xc4216c4580, 0xc691e0, 0xc4214277e0, 0x10000, 0x7fffffff0000, 0x66e59d, 0xc421b1fe48)
	pkg/sentry/mm/vma.go:338 +0x1a4
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).unmapLocked(0xc4216c4580, 0xc691e0, 0xc4214277e0, 0x10000, 0x7fffffff0000, 0x18, 0xdb)
	pkg/sentry/mm/vma.go:310 +0x7e
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).DecUsers(0xc4216c4580, 0xc691e0, 0xc4214277e0)
	pkg/sentry/mm/lifecycle.go:220 +0x1dd
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*TaskContext).release(0xc423c94b30)
	pkg/sentry/kernel/task_context.go:64 +0xd6
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*runExitMain).execute(0x0, 0xc423c94400, 0xc5bc00, 0x0)
	pkg/sentry/kernel/task_exit.go:266 +0xc5
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).run(0xc423c94400, 0x4bc1)
	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