ob-plantuml: explicitly check `org-plantuml-jar-path' before use

* lisp/ob-plantuml.el (org-babel-execute:plantuml): explicitly check
  `org-plantuml-jar-path' before use

lisp/ob-plantuml.el

@@ -57,12 +57,14 @@ This function is called by `org-babel-execute-src-block'."
 	 (out-file (cdr (assoc :file params)))
 	 (cmdline (cdr (assoc :cmdline params)))
 	 (in-file (org-babel-temp-file "plantuml-"))
-	 (cmd (concat "java -jar "
-		      (shell-quote-argument org-plantuml-jar-path)
-		      " -p " cmdline " < "
-		      (shell-quote-argument in-file)
-		      " > "
-		      (shell-quote-argument out-file))))
+	 (cmd (if (not org-plantuml-jar-path)
+		  (error "`org-plantuml-jar-path' is not set")
+		(concat "java -jar "
+			(shell-quote-argument org-plantuml-jar-path)
+			" -p " cmdline " < "
+			(shell-quote-argument in-file)
+			" > "
+			(shell-quote-argument out-file)))))
     (unless (file-exists-p org-plantuml-jar-path)
       (error "Could not find plantuml.jar at %s" org-plantuml-jar-path))
     (with-temp-file in-file (insert (concat "@startuml\n" body "\n@enduml")))