CLEANUP: conn: Do not pass a pointer to likely

Move the `!` inside the likely and negate it to unlikely.

The previous version should not have caused issues, because it is converted
to a boolean / integral value before being passed to __builtin_expect(), but
it's certainly unusual.

[wt: this was not a bug but purposely written like this to improve code
 generation on older compilers but not needed anymore as described here:
 https://www.mail-archive.com/haproxy@formilux.org/msg36392.html ]
diff --git a/include/proto/connection.h b/include/proto/connection.h
index c7caeea..fb264d2 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -400,12 +400,12 @@
 	struct conn_stream *cs;
 
 	cs = pool_alloc(pool_head_connstream);
-	if (!likely(cs))
+	if (unlikely(!cs))
 		return NULL;
 
 	if (!conn) {
 		conn = conn_new();
-		if (!likely(conn)) {
+		if (unlikely(!conn)) {
 			cs_free(cs);
 			return NULL;
 		}