fix(mtcp,mws,mtls): close dead session's base conn before dropping it

Dial() deleted a dead muxSession without closing session.conn, leaking
the base TCP conn on every dead session (and, once a p2p base conn is
injected, leaking the plugin-side tunnel). session.Close() is a no-op
when session.session is nil, so the base conn is closed explicitly.

dialer/mtcp/dialer.go

@@ -61,6 +61,10 @@ func (d *mtcpDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialO
 
 	session, ok := d.sessions[addr]
 	if session != nil && session.IsClosed() {
+		session.Close()
+		if session.conn != nil {
+			session.conn.Close() // base conn would otherwise leak (and hold a p2p tunnel open)
+		}
 		delete(d.sessions, addr) // session is dead
 		ok = false
 	}

dialer/mtls/dialer.go

@@ -62,6 +62,10 @@ func (d *mtlsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialO
 
 	session, ok := d.sessions[addr]
 	if session != nil && session.IsClosed() {
+		session.Close()
+		if session.conn != nil {
+			session.conn.Close() // base conn would otherwise leak (and hold a p2p tunnel open)
+		}
 		delete(d.sessions, addr) // session is dead
 		ok = false
 	}

dialer/mws/dialer.go

@@ -76,6 +76,10 @@ func (d *mwsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOp
 
 	session, ok := d.sessions[addr]
 	if session != nil && session.IsClosed() {
+		session.Close()
+		if session.conn != nil {
+			session.conn.Close() // base conn would otherwise leak (and hold a p2p tunnel open)
+		}
 		delete(d.sessions, addr) // session is dead
 		ok = false
 	}