Guard evaluation of server-use-tcp because it might not be defined

Thanks to Nicolas Dudebout for the suggestion.
Closes magitGH-283

magit.el

@@ -4997,7 +4997,8 @@ Return values:
   (if (functionp 'server-running-p)
       (server-running-p)
     (condition-case nil
-	(if server-use-tcp
+	(if (and (boundp 'server-use-tcp)
+                 server-use-tcp)
 	    (with-temp-buffer
 	      (insert-file-contents-literally (expand-file-name server-name server-auth-dir))
 	      (or (and (looking-at "127\\.0\\.0\\.1:[0-9]+ \\([0-9]+\\)")