only try to insert extant hashes

* lisp/ob-lob.el (org-babel-lob-execute): Only try to insert extant
  hashes.

lisp/ob-lob.el

@@ -139,7 +139,7 @@ if so then run the appropriate source block from the Library."
 	  (prog1 (org-babel-execute-src-block
 		  nil (mkinfo (org-babel-process-params pre-params)))
 	    ;; update the hash
-	    (org-babel-set-current-result-hash new-hash))))))
+	    (when hash (org-babel-set-current-result-hash new-hash)))))))
 
 (provide 'ob-lob)