MINOR: cli: allow to set frontend maxconn to zero

It is sometimes useful to completely disable accepting new connections
on a frontend during maintenance operations. By setting a frontend's
maxconn to zero, connections are not accepted anymore until the limit
is increased again.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index 7c01e79..c8b60a6 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -1113,8 +1113,7 @@
 				}
 
 				v = atoi(args[4]);
-				/* check for unlimited values, we restore default setting (cfg_maxpconn) */
-				if (v < 1) {
+				if (v < 0) {
 					si->applet.ctx.cli.msg = "Value out of range.\n";
 					si->applet.st0 = STAT_CLI_PRINT;
 					return 1;