fix(session): fix panic: close of closed channel

[oiooj]

This new version breaks our internal service, the panic stack:

panic: close of closed channel

goroutine 41 [running]:
github.com/grafana/pyroscope-go.(*cpuProfileCollector).Stop(0x1400018e090)
	/tmp/pyroscope-go/collector.go:181 +0xa0
github.com/grafana/pyroscope-go.(*Session).takeSnapshots(0x14000192000)
	/tmp/pyroscope-go/session.go:164 +0x21c
github.com/grafana/pyroscope-go.(*Session).Start.func1()
	/tmp/pyroscope-go/session.go:183 +0x54
created by github.com/grafana/pyroscope-go.(*Session).Start in goroutine 35
	/tmp/pyroscope-go/session.go:181 +0x98
exit status 2
FAIL	github.com/grafana/pyroscope-go	0.221s

@korniltsev

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[korniltsev]

lgtm

[korniltsev]

cc @kolesnikovae

[kolesnikovae]

LGTM

Thanks for the fix @oiooj!