wgengine/magicsock: fix data race with sync.Pool in error+logging path

Fixes #3122

Change-Id: Ib52e84f9bd5813d6cf2e80ce5b2296912a48e064
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit c759fcc7d3)

wgengine/magicsock/magicsock.go

@@ -1149,8 +1149,12 @@ var udpAddrPool = &sync.Pool{
 // See sendAddr's docs on the return value meanings.
 func (c *Conn) sendUDP(ipp netaddr.IPPort, b []byte) (sent bool, err error) {
 	ua := udpAddrPool.Get().(*net.UDPAddr)
-	defer udpAddrPool.Put(ua)
-	return c.sendUDPStd(ipp.UDPAddrAt(ua), b)
+	sent, err = c.sendUDPStd(ipp.UDPAddrAt(ua), b)
+	if err == nil {
+		// Only return it to the pool on success; Issue 3122.
+		udpAddrPool.Put(ua)
+	}
+	return
 }
 
 // sendUDP sends UDP packet b to addr.