Cleanup encoding pointer comparison

* parser.c: Compare encoding pointer with a NULL instead of
  xmlCharEncoding enum value 0 then casted to char * !
diff --git a/parser.c b/parser.c
index 1db706b..e8cd123 100644
--- a/parser.c
+++ b/parser.c
@@ -6609,7 +6609,7 @@
     xmlDetectSAX2(ctxt);
     GROW;
 
-    if ((ctxt->encoding == (const xmlChar *)XML_CHAR_ENCODING_NONE) &&
+    if ((ctxt->encoding == NULL) &&
         (ctxt->input->end - ctxt->input->cur >= 4)) {
         xmlChar start[4];
 	xmlCharEncoding enc;
@@ -10105,7 +10105,7 @@
     if ((ctxt->sax) && (ctxt->sax->setDocumentLocator))
         ctxt->sax->setDocumentLocator(ctxt->userData, &xmlDefaultSAXLocator);
 
-    if ((ctxt->encoding == (const xmlChar *)XML_CHAR_ENCODING_NONE) &&
+    if ((ctxt->encoding == NULL) &&
         ((ctxt->input->end - ctxt->input->cur) >= 4)) {
 	/* 
 	 * Get the 4 first bytes and decode the charset