make sure xmllint --stream and xmllint --stream --valid returns errors

* xmllint.c xmlreader.c include/libxml/xmlreader.h: make sure
  xmllint --stream and xmllint --stream --valid returns errors
  code appropriately
Daniel
diff --git a/xmlreader.c b/xmlreader.c
index 529ef56..4821b2a 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -2612,6 +2612,20 @@
 }
 
 /**
+ * xmlTextReaderIsValid:
+ * @reader:  the xmlTextReaderPtr used
+ *
+ * Retrieve the validity status from the parser context
+ *
+ * Returns the flag value 1 if valid, 0 if no, and -1 in case of error
+ */
+int
+xmlTextReaderIsValid(xmlTextReaderPtr reader) {
+    if ((reader == NULL) || (reader->ctxt == NULL)) return(-1);
+    return(reader->ctxt->valid);
+}
+
+/**
  * xmlTextReaderGetErrorHandler:
  * @reader:  the xmlTextReaderPtr used
  * @f:	the callback function or NULL is no callback has been registered