panic: Decrementing non-positive ref count

goroutine 30 [running]:
gvisor.googlesource.com/gvisor/pkg/refs.(*AtomicRefCount).DecRefWithDestructor(0xc4203de0c0, 0xc42038bd58)
	pkg/refs/refcounter.go:267 +0x162
gvisor.googlesource.com/gvisor/pkg/sentry/kernel/shm.(*Shm).DecRef(0xc4203de0c0)
	pkg/sentry/kernel/shm/shm.go:373 +0x48
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).removeVMAsLocked(0xc42043cc80, 0xc75c20, 0xc42047a5a0, 0x10000, 0x7fffffff0000, 0x67af1d, 0xc42038be48)
	pkg/sentry/mm/vma.go:338 +0x1a4
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).unmapLocked(0xc42043cc80, 0xc75c20, 0xc42047a5a0, 0x10000, 0x7fffffff0000, 0x18, 0x0)
	pkg/sentry/mm/vma.go:310 +0x7e
gvisor.googlesource.com/gvisor/pkg/sentry/mm.(*MemoryManager).DecUsers(0xc42043cc80, 0xc75c20, 0xc42047a5a0)
	pkg/sentry/mm/lifecycle.go:220 +0x1dd
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*TaskContext).release(0xc4204f3030)
	pkg/sentry/kernel/task_context.go:62 +0xd6
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*runExitMain).execute(0x0, 0xc4204f2900, 0xc68a20, 0x0)
	pkg/sentry/kernel/task_exit.go:261 +0xc5
gvisor.googlesource.com/gvisor/pkg/sentry/kernel.(*Task).run(0xc4204f2900, 0x7)
	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:251 +0x100