net/http: fix ServeMux pattern registration

When the httpmuxgo121 GODEBUG setting was active, we were registering
patterns in the old and the new way. Fix to register only in the old
way.

Change-Id: Ibc1fd41e7f4d162ee5bc34575df409e1db5657cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/533095
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Olena Synenka <olenasynenka@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/server.go

@@ -2686,8 +2686,9 @@ func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
 func (mux *ServeMux) Handle(pattern string, handler Handler) {
 	if use121 {
 		mux.mux121.handle(pattern, handler)
+	} else {
+		mux.register(pattern, handler)
 	}
-	mux.register(pattern, handler)
 }
 
 // HandleFunc registers the handler function for the given pattern.
@@ -2696,26 +2697,29 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
 	if use121 {
 		mux.mux121.handleFunc(pattern, handler)
+	} else {
+		mux.register(pattern, HandlerFunc(handler))
 	}
-	mux.register(pattern, HandlerFunc(handler))
 }
 
 // Handle registers the handler for the given pattern in [DefaultServeMux].
 // The documentation for [ServeMux] explains how patterns are matched.
 func Handle(pattern string, handler Handler) {
 	if use121 {
 		DefaultServeMux.mux121.handle(pattern, handler)
+	} else {
+		DefaultServeMux.register(pattern, handler)
 	}
-	DefaultServeMux.register(pattern, handler)
 }
 
 // HandleFunc registers the handler function for the given pattern in [DefaultServeMux].
 // The documentation for [ServeMux] explains how patterns are matched.
 func HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
 	if use121 {
 		DefaultServeMux.mux121.handleFunc(pattern, handler)
+	} else {
+		DefaultServeMux.register(pattern, HandlerFunc(handler))
 	}
-	DefaultServeMux.register(pattern, HandlerFunc(handler))
 }
 
 func (mux *ServeMux) register(pattern string, handler Handler) {