Fix to bug #673, http://bugs.skysql.com/show_bug.cgi?id=673

dcb.c:dcb_hashtable_stats:division by zero
hashtable.c:hashtable_get_stats: NULL-pointer reference
service.c:serviceStartPort:set service->users NULL to avoid referring to freed memory
users.c:dcb_usersPrintf: NULL-pointer reference
debugcmd.c:convert_arg: changed return value to 1 in case of error, 0 (==NULL) is valid but it indicates that there are no users loaded.
	execute_cmd: fixed command handling

server/core/dcb.c

@@ -1610,7 +1610,9 @@ void dcb_hashtable_stats(
                    hashsize);
         
 	dcb_printf(dcb, "\tNo. of entries:     	%d\n", total);
-	dcb_printf(dcb, "\tAverage chain length:	%.1f\n", (float)total / hashsize);
+	dcb_printf(dcb, 
+		"\tAverage chain length:	%.1f\n", 
+		(hashsize == 0 ? (float)hashsize : (float)total / hashsize));
 	dcb_printf(dcb, "\tLongest chain length:	%d\n", longest);
 }