BUG/MEDIUM: connections: Don't try to send early data if we have no mux.

In connect_server(), if we don't yet have a mux, because we're choosing
one depending on the ALPN, don't attempt to send early data. We can't do
it because those data would depend on the mux, that will only be determined
by the handshake.

This should be backported to 1.9.
diff --git a/src/backend.c b/src/backend.c
index 809a323..bff71ab 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1605,7 +1605,7 @@
 
 #if USE_OPENSSL && (defined(OPENSSL_IS_BORINGSSL) || (HA_OPENSSL_VERSION_NUMBER >= 0x10101000L))
 
-	if (!reuse && cli_conn && srv &&
+	if (!reuse && cli_conn && srv && srv_conn->mux &&
 	    (srv->ssl_ctx.options & SRV_SSL_O_EARLY_DATA) &&
 	    /* Only attempt to use early data if either the client sent
 	     * early data, so that we know it can handle a 425, or if