BUG/MINOR: listener: add an error check for unallocatable trash

Coverity noticed in issue #1416 that a missing allocation error was
introduced in tcp_bind_listener() with the rework of error messages by
commit ed1748553 ("MINOR: proto_tcp: use chunk_appendf() to ouput socket
setup errors"). In practice nobody will ever face it but better address
it anyway.

No backport is needed.

src/proto_tcp.c

@@ -589,6 +589,12 @@ int tcp_bind_listener(struct listener *listener, char *errmsg, int errlen)
 
 	err = ERR_NONE;
 
+	if (!msg) {
+		if (errlen)
+			snprintf(errmsg, errlen, "out of memory");
+		return ERR_ALERT | ERR_FATAL;
+	}
+
 	/* ensure we never return garbage */
 	if (errlen)
 		*errmsg = 0;