[MINOR] recompute maxfd before touching fdtab

It may be dangerous to play with fdtab before doing fd_insert()
because this last one is responsible for growing maxfd as needed.
Call fd_insert() before instead.
diff --git a/src/proxy.c b/src/proxy.c
index a5431c2..93eab16 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -147,12 +147,12 @@
 			listener->fd = fd;
 
 			/* the function for the accept() event */
+			fd_insert(fd);
 			fdtab[fd].cb[DIR_RD].f  = &event_accept;
 			fdtab[fd].cb[DIR_WR].f = NULL; /* never called */
 			fdtab[fd].cb[DIR_RD].b = fdtab[fd].cb[DIR_WR].b = NULL;
 			fdtab[fd].owner = (struct task *)curproxy; /* reference the proxy instead of a task */
 			fdtab[fd].state = FD_STLISTEN;
-			fd_insert(fd);
 			listeners++;
 		}