entities: Report malloc failures

Fix places where malloc failures aren't reported.

Introduce new API function xmlAddEntity that returns separate error
codes.

Don't invoke global error handler for low-level errors which should be
handled by higher layers.

Invalid redelcaration warnings will be fixed later.

include/libxml/entities.h

@@ -87,6 +87,15 @@ XMLPUBFUN xmlEntityPtr
 						 const xmlChar *content);
 XMLPUBFUN void
 			xmlFreeEntity		(xmlEntityPtr entity);
+XMLPUBFUN int
+			xmlAddEntity		(xmlDocPtr doc,
+						 int extSubset,
+						 const xmlChar *name,
+						 int type,
+						 const xmlChar *ExternalID,
+						 const xmlChar *SystemID,
+						 const xmlChar *content,
+						 xmlEntityPtr *out);
 XMLPUBFUN xmlEntityPtr
 			xmlAddDocEntity		(xmlDocPtr doc,
 						 const xmlChar *name,