http: fix panic when recovering from hijacked connection panic

Fixes #2375.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5279049

src/pkg/http/server.go

@@ -567,7 +567,9 @@ func (c *conn) serve() {
 		if err == nil {
 			return
 		}
+		if c.rwc != nil { // may be nil if connection hijacked
 			c.rwc.Close()
+		}
 
 		var buf bytes.Buffer
 		fmt.Fprintf(&buf, "http: panic serving %v: %v\n", c.remoteAddr, err)