Re-enable CHECK() than only 0-numbered objects are released.

Review-Url: https://chromiumcodereview.appspot.com/2424933002
diff --git a/core/fpdfapi/parser/cpdf_object.cpp b/core/fpdfapi/parser/cpdf_object.cpp
index 3bf0a95..5c40733 100644
--- a/core/fpdfapi/parser/cpdf_object.cpp
+++ b/core/fpdfapi/parser/cpdf_object.cpp
@@ -38,9 +38,7 @@
 }
 
 void CPDF_Object::Release() {
-  if (m_ObjNum)
-    return;
-
+  CHECK(!m_ObjNum);
   delete this;
 }