reader: Fix return value of xmlTextReaderReadString again

Make sure to return NULL for node types except elements or text to match
the old behavior.

Note that CDATA sections are still treated like text nodes and will have
their content returned.

Fixes #838.

testparser.c

@@ -467,6 +467,76 @@ testReaderContent(void) {
     return err;
 }
 
+static int
+testReaderNode(xmlTextReader *reader) {
+    xmlChar *string;
+    int type;
+    int err = 0;
+
+    type = xmlTextReaderNodeType(reader);
+    string = xmlTextReaderReadString(reader);
+
+    if (type == XML_READER_TYPE_ELEMENT) {
+        xmlNodePtr node = xmlTextReaderCurrentNode(reader);
+
+        if ((node->children == NULL) != (string == NULL))
+            err = 1;
+    } else if (type == XML_READER_TYPE_TEXT ||
+               type == XML_READER_TYPE_CDATA ||
+               type == XML_READER_TYPE_WHITESPACE ||
+               type == XML_READER_TYPE_SIGNIFICANT_WHITESPACE) {
+        if (string == NULL)
+            err = 1;
+    } else {
+        if (string != NULL)
+            err = 1;
+    }
+
+    if (err)
+        fprintf(stderr, "xmlTextReaderReadString failed for %d\n", type);
+
+    xmlFree(string);
+
+    return err;
+}
+
+static int
+testReader(void) {
+    xmlTextReader *reader;
+    const xmlChar *xml = BAD_CAST
+        "<d>\n"
+        "  x<e a='v'>y</e><f>z</f>\n"
+        "  <![CDATA[cdata]]>\n"
+        "  <!-- comment -->\n"
+        "  <?pi content?>\n"
+        "  <empty/>\n"
+        "</d>";
+    int err = 0;
+
+    reader = xmlReaderForDoc(xml, NULL, NULL, 0);
+
+    while (xmlTextReaderRead(reader) > 0) {
+        if (testReaderNode(reader) > 0) {
+            err = 1;
+            break;
+        }
+
+        if (xmlTextReaderMoveToFirstAttribute(reader) > 0) {
+            do {
+                if (testReaderNode(reader) > 0) {
+                    err = 1;
+                    break;
+                }
+            } while (xmlTextReaderMoveToNextAttribute(reader) > 0);
+
+            xmlTextReaderMoveToElement(reader);
+        }
+    }
+
+    xmlFreeTextReader(reader);
+    return err;
+}
+
 #ifdef LIBXML_XINCLUDE_ENABLED
 typedef struct {
     char *message;
@@ -834,6 +904,7 @@ main(void) {
 #ifdef LIBXML_READER_ENABLED
     err |= testReaderEncoding();
     err |= testReaderContent();
+    err |= testReader();
 #ifdef LIBXML_XINCLUDE_ENABLED
     err |= testReaderXIncludeError();
 #endif