net/rpc: don't assume b.N > 0

Change-Id: I58c4a75168fd1f797a25735c4151f501f0475332
Reviewed-on: https://go-review.googlesource.com/20854Reviewed-by: Russ Cox <rsc@golang.org>

src/net/rpc/server_test.go

@@ -657,6 +657,9 @@ func benchmarkEndToEnd(dial func() (*Client, error), b *testing.B) {
 }
 
 func benchmarkEndToEndAsync(dial func() (*Client, error), b *testing.B) {
+	if b.N == 0 {
+		return
+	}
 	const MaxConcurrentCalls = 100
 	once.Do(startServer)
 	client, err := dial()