Fix detached datachannels handling

#2696 introduced removing
datachannels from the sctptransport for better garbage collection.

That PR introduced a race condition for data channels created before
connection establishment. When an out of band negotiated data channel,
created before peerconnection establishment is detached, there's a race
between the data channel being removed from `r.dataChannels` and it
being copied in to the existing data channel slice in the
acceptDataChannels goroutine.

This PR fixes this race by copying the slice before any datachannels
could be detached.

sctptransport.go

@@ -142,7 +142,7 @@ func (r *SCTPTransport) Start(_ SCTPCapabilities) error {
 	r.dataChannelsOpened += openedDCCount
 	r.lock.Unlock()
 
-	go r.acceptDataChannels(sctpAssociation)
+	go r.acceptDataChannels(sctpAssociation, dataChannels)
 
 	return nil
 }
@@ -163,10 +163,9 @@ func (r *SCTPTransport) Stop() error {
 	return nil
 }
 
-func (r *SCTPTransport) acceptDataChannels(a *sctp.Association) {
-	r.lock.RLock()
-	dataChannels := make([]*datachannel.DataChannel, 0, len(r.dataChannels))
-	for _, dc := range r.dataChannels {
+func (r *SCTPTransport) acceptDataChannels(a *sctp.Association, existingDataChannels []*DataChannel) {
+	dataChannels := make([]*datachannel.DataChannel, 0, len(existingDataChannels))
+	for _, dc := range existingDataChannels {
 		dc.mu.Lock()
 		isNil := dc.dataChannel == nil
 		dc.mu.Unlock()
@@ -175,7 +174,6 @@ func (r *SCTPTransport) acceptDataChannels(a *sctp.Association) {
 		}
 		dataChannels = append(dataChannels, dc.dataChannel)
 	}
-	r.lock.RUnlock()
 
 ACCEPT:
 	for {