[bugfix] Avoid NPE in let expression.
[bugfix] Avoid NPE in let expression.

svn path=/trunk/eXist/; revision=8058

hungerburg/exist,dizzzz/exist,opax/exist,wolfgangmm/exist,olvidalo/exist,olvidalo/exist,MjAbuz/exist,shabanovd/exist,dizzzz/exist,hungerburg/exist,eXist-db/exist,ljo/exist,eXist-db/exist,kohsah/exist,ambs/exist,MjAbuz/exist,ambs/exist,wolfgangmm/exist,RemiKoutcherawy/exist,shabanovd/exist,wshager/exist,joewiz/exist,eXist-db/exist,lcahlander/exist,wolfgangmm/exist,adamretter/exist,windauer/exist,opax/exist,kohsah/exist,wshager/exist,patczar/exist,joewiz/exist,windauer/exist,wshager/exist,RemiKoutcherawy/exist,joewiz/exist,wolfgangmm/exist,jensopetersen/exist,dizzzz/exist,ljo/exist,RemiKoutcherawy/exist,adamretter/exist,kohsah/exist,olvidalo/exist,MjAbuz/exist,RemiKoutcherawy/exist,dizzzz/exist,zwobit/exist,jessealama/exist,wshager/exist,patczar/exist,windauer/exist,zwobit/exist,adamretter/exist,hungerburg/exist,RemiKoutcherawy/exist,zwobit/exist,MjAbuz/exist,eXist-db/exist,adamretter/exist,wshager/exist,jessealama/exist,wshager/exist,shabanovd/exist,jensopetersen/exist,hungerburg/exist,ljo/exist,jensopetersen/exist,shabanovd/exist,opax/exist,jensopetersen/exist,zwobit/exist,adamretter/exist,kohsah/exist,windauer/exist,zwobit/exist,patczar/exist,ambs/exist,opax/exist,olvidalo/exist,dizzzz/exist,joewiz/exist,windauer/exist,MjAbuz/exist,ambs/exist,dizzzz/exist,shabanovd/exist,windauer/exist,eXist-db/exist,lcahlander/exist,hungerburg/exist,joewiz/exist,patczar/exist,joewiz/exist,ambs/exist,ljo/exist,jensopetersen/exist,ambs/exist,RemiKoutcherawy/exist,jessealama/exist,ljo/exist,lcahlander/exist,jessealama/exist,ljo/exist,jessealama/exist,lcahlander/exist,shabanovd/exist,wolfgangmm/exist,lcahlander/exist,eXist-db/exist,patczar/exist,kohsah/exist,adamretter/exist,patczar/exist,jensopetersen/exist,lcahlander/exist,jessealama/exist,kohsah/exist,MjAbuz/exist,wolfgangmm/exist,opax/exist,olvidalo/exist,zwobit/exist