Bug 1142503 - don't use QueryInterface when the compiler can do the cast for us; r=ehsan

Calling QueryInterface with a statically known IID should typically not
be necessary.  In those cases where it's not, the compiler can do the
cast for us, though we have to supply the reference-counting that
QueryInterface would do.

In passing, several redundant null-checks for the result of |new T| have
been deleted.

editor/composer/nsComposerCommandsUpdater.cpp

@@ -385,9 +385,7 @@ nsComposerCommandsUpdater::Notify(nsITimer *timer)
 nsresult
 NS_NewComposerCommandsUpdater(nsISelectionListener** aInstancePtrResult)
 {
-  nsComposerCommandsUpdater* newThang = new nsComposerCommandsUpdater;
-  NS_ENSURE_TRUE(newThang, NS_ERROR_OUT_OF_MEMORY);
-
-  return newThang->QueryInterface(NS_GET_IID(nsISelectionListener),
-                                  (void **)aInstancePtrResult);
+  nsRefPtr<nsComposerCommandsUpdater> newThang = new nsComposerCommandsUpdater;
+  newThang.forget(aInstancePtrResult);
+  return NS_OK;
 }