GetProtocols memory implementation incorrect slice preallocation

[mrd0ll4r]

A small performance bug:

The memory implementation holds segments of protocols map[peer.ID]map[string]struct{}.
In GetProtocols(p peer.ID) a slice is preallocated to len((segment).protocols), which is the number of peers in that segment.
It was probably intended to be be len((segment).protocols[p]) instead.

Source: https://github.com/libp2p/go-libp2p-peerstore/blob/master/pstoremem/protobook.go#L134

Fix:

s := pb.segments.get(p)
s.RLock()
defer s.RUnlock()

out := make([]string, 0, len(s.protocols[p]))
for k := range s.protocols[p] {
	out = append(out, k)
}

[marten-seemann]

Good catch! Would you mind submitting a PR?