MINOR: listeners: make the accept loop more robust when maxaccept==0

If some listeners are mistakenly configured with 0 as the maxaccept value,
then we now consider them as limited to one accept() at a time. This will
avoid some issues as fixed by the past commit.

src/listener.c

@@ -252,7 +252,7 @@ void listener_accept(int fd)
 {
 	struct listener *l = fdtab[fd].owner;
 	struct proxy *p = l->frontend;
-	int max_accept = l->maxaccept;
+	int max_accept = l->maxaccept ? l->maxaccept : 1;
 	int cfd;
 	int ret;