panic: Decrementing non-positive ref count

goroutine 1986718 [running]:
gvisor.googlesource.com/gvisor/pkg/refs.(*AtomicRefCount).DecRefWithDestructor(0xc420365200, 0xc42217bca0)
	pkg/refs/refcounter.go:271 +0x1d9
gvisor.googlesource.com/gvisor/pkg/sentry/kernel/shm.(*Shm).DecRef(0xc420365200)
	pkg/sentry/kernel/shm/shm.go:377 +0x7a
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).removeVMAsLocked(0xc4218ac000, 0x10cbc20, 0xc421f69e20, 0x10000, 0x7fffffff0000, 0xc4207a1230, 0xc4218ac05c)
	pkg/sentry/mm/vma.go:338 +0x308
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).unmapLocked(0xc4218ac000, 0x10cbc20, 0xc421f69e20, 0x10000, 0x7fffffff0000, 0xc42217be68, 0xc421f69e20)
	pkg/sentry/mm/vma.go:310 +0x8c
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).DecUsers(0xc4218ac000, 0x10cbc20, 0xc421f69e20)
	pkg/sentry/mm/lifecycle.go:220 +0x298
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*TaskContext).release(0xc421f56730)
	pkg/sentry/kernel/task_context.go:64 +0xe6
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*runExitMain).execute(0x0, 0xc421f56000, 0x10bc5e0, 0x0)
	pkg/sentry/kernel/task_exit.go:266 +0x153
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).run(0xc421f56000, 0x6040)
	pkg/sentry/kernel/task_run.go:95 +0x265
created by gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).Start
	pkg/sentry/kernel/task_start.go:258 +0x194