Don't try to register command line handler if already registered

Avoids an error when reiniting during tests

chrome/content/zotero/xpcom/commandLineHandler.js

@@ -130,6 +130,8 @@ var ZoteroCommandLineHandler = {
 };
 
 const Cm = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+// Don't register if already registered (e.g., after a reinit() in tests)
+if (!Cm.isCIDRegistered(ZoteroCommandLineHandler.classID)) {
 	Cm.registerFactory(
 		ZoteroCommandLineHandler.classID,
 		"command-line-handler",
@@ -141,3 +143,4 @@ const catman = Cc["@mozilla.org/categorymanager;1"].getService(Ci.nsICategoryMan
 	catman.addCategoryEntry("command-line-handler",
 		"m-zotero",
 		ZoteroCommandLineHandler.contractID, false, true);
+}