un-duplicate ditaa eps file name generation

* lisp/ob-ditaa.el (org-babel-execute:ditaa): Un-duplicate ditaa eps
  file name generation.

lisp/ob-ditaa.el

@@ -90,11 +90,13 @@ This function is called by `org-babel-execute-src-block'."
 	 (java (cdr (assoc :java params)))
 	 (in-file (org-babel-temp-file "ditaa-"))
 	 (eps (cdr (assoc :eps params)))
+	 (eps-file (when eps
+		     (org-babel-process-file-name (concat in-file ".eps"))))
 	 (pdf-cmd (when (and (or (string= (file-name-extension out-file) "pdf")
 				 (cdr (assoc :pdf params))))
 		    (concat
 		     "epstopdf"
-		     " " (org-babel-process-file-name (concat in-file ".eps"))
+		     " " eps-file
 		     " -o=" (org-babel-process-file-name out-file))))
 	 (cmd (concat org-babel-ditaa-java-cmd
 		      " " java " " org-ditaa-jar-option " "
@@ -104,7 +106,7 @@ This function is called by `org-babel-execute-src-block'."
 		      " " cmdline
 		      " " (org-babel-process-file-name in-file)
 		      " " (if pdf-cmd
-			      (org-babel-process-file-name (concat in-file ".eps"))
+			      eps-file
 			    (org-babel-process-file-name out-file)))))
     (unless (file-exists-p org-ditaa-jar-path)
       (error "Could not find ditaa.jar at %s" org-ditaa-jar-path))