Fixed issue in CMISURLUtil that can cause a crash.

git-svn-id: https://svn.apache.org/repos/asf/chemistry/objectivecmis/trunk@1644864 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/ObjectiveCMIS/Utils/CMISURLUtil.m b/ObjectiveCMIS/Utils/CMISURLUtil.m
index c170f43..e84ad19 100644
--- a/ObjectiveCMIS/Utils/CMISURLUtil.m
+++ b/ObjectiveCMIS/Utils/CMISURLUtil.m
@@ -38,7 +38,7 @@
 
 + (NSString *)urlStringByAppendingParameter:(NSString *)parameterName value:(NSString *)parameterValue urlString:(NSString *)urlString
 {
-    if (parameterName == nil || parameterValue == nil) {
+    if (parameterName == nil || parameterValue == nil || urlString == nil) {
         return urlString;
     }