tris: restore retry logic on warning alerts

Not sure why the retry logic was removed since 0-RTT works even in
presence of the special alerts handling (alertEndOfEarlyData is
processed before the warning alert). To reduce divergence from upstream
code (which adds a restriction on the number of consecutive warnings),
restore the original retry logic.

Do not do anything fancy here, later drafts will remove the special
alert handling since it becomes a special handshake message.

Fixes: ("crypto/tls: implement TLS 1.3 server 0-RTT")

conn.go

@@ -649,6 +649,7 @@ func (c *Conn) readRecord(want recordType) error {
 		}
 	}
 
+Again:
 	if c.rawInput == nil {
 		c.rawInput = c.in.newBlock()
 	}
@@ -776,7 +777,7 @@ func (c *Conn) readRecord(want recordType) error {
 		case alertLevelWarning:
 			// drop on the floor
 			c.in.freeBlock(b)
-			return nil
+			goto Again
 		case alertLevelError:
 			c.in.setErrorLocked(&net.OpError{Op: "remote error", Err: alert(data[1])})
 		default: