trace: fix data race in RenderEvents

This commit updates the RenderEvents function by moving the read
lock for event families so that it happens before checking the
length of event families and by that fixing a data race problem.

Fixes golang/go#76402

Change-Id: Id02ebaa25cec8ef631aaf928ba084b0692e84583
Reviewed-on: https://go-review.googlesource.com/c/net/+/723160
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>

Author: marefr

trace/events.go

@@ -58,8 +58,8 @@ func RenderEvents(w http.ResponseWriter, req *http.Request, sensitive bool) {
 		Buckets: buckets,
 	}
 
-	data.Families = make([]string, 0, len(families))
 	famMu.RLock()
+	data.Families = make([]string, 0, len(families))
 	for name := range families {
 		data.Families = append(data.Families, name)
 	}