Fix issues with unaligned atomic operations.

This could be solved by simply reordering the fields, but it
is more robust to move the atomics into their own structure,
and the extra indirection should not cost much.

rtpconn/rtpwriter.go

@@ -266,12 +266,12 @@ func rtpWriterLoop(writer *rtpWriter, up *rtpUpConnection, track *rtpUpTrack) {
 				track.mu.Lock()
 				ntp := track.srNTPTime
 				rtp := track.srRTPTime
-				cname := track.cname
 				track.mu.Unlock()
 				if ntp != 0 {
 					action.track.SetTimeOffset(ntp, rtp)
 				}
-				if cname != "" {
+				cname, ok := track.cname.Load().(string)
+				if ok && cname != "" {
 					action.track.SetCname(cname)
 				}