net: fix socket leak in case of Dial failure

Socket descriptors are not closed when fd.connect() fails during generic socket creation.
After a connection failure [ECONNREFUSED] descriptors are left in SYN_SENT state indefinitely (unless they get an explicit RST). Repeated failed connections will eventually cause your program to hit the user/system max-open-files limit.

Fixes #2349.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5229047

src/pkg/net/sock.go

@@ -52,6 +52,7 @@ func socket(net string, f, p, t int, la, ra syscall.Sockaddr, toAddr func(syscal
 
 	if ra != nil {
 		if err = fd.connect(ra); err != nil {
+			closesocket(s)
 			fd.Close()
 			return nil, err
 		}