net/http: delete unnecessary string concatenation

In the existing implementation, if pattern is an empty string,
program calls a panic with the message which is a concatenation of
"http: invalid pattern " and pattern.
In this case, pattern is an empty, so the commit removes
this concatenation and the trailing space.

Fixes: #21102

Change-Id: I49f58b52d835311a6ac642de871eb15646e48a54
Reviewed-on: https://go-review.googlesource.com/50350Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/server.go

@@ -2265,7 +2265,7 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 	defer mux.mu.Unlock()
 
 	if pattern == "" {
-		panic("http: invalid pattern " + pattern)
+		panic("http: invalid pattern")
 	}
 	if handler == nil {
 		panic("http: nil handler")