fix(ldap): properly escape ldap input

SoObjects/SOGo/SOGoSession.m

@@ -48,7 +48,7 @@
   //We check that the session is base64
   NSData *decodedData = [[NSData alloc] initWithBase64EncodedString:theSessionKey options:0];
   if(!decodedData)
-    return value;
+    return nil;
 
   key = [NSString stringWithFormat: @"session:%@", theSessionKey];
   value = [cache valueForKey: key];