valid: Improve addition and deletion of IDs

Introduce a new API function xmlAddIDSafe that returns a separate error
code if a memory allocation fails.

Store a pointer to the ID struct in xmlAttr so attributes can be
freed without allocating memory. It's impossible to report malloc
failures in deallocation code.

include/libxml/valid.h

@@ -249,6 +249,12 @@ XMLPUBFUN void
 #endif /* LIBXML_OUTPUT_ENABLED */
 
 /* IDs */
+XMLPUBFUN int
+		xmlAddIDSafe	       (xmlDocPtr doc,
+					const xmlChar *value,
+					xmlAttrPtr attr,
+					int streaming,
+					xmlIDPtr *id);
 XMLPUBFUN xmlIDPtr
 		xmlAddID	       (xmlValidCtxtPtr ctxt,
 					xmlDocPtr doc,