xmlreader: Fix return value of xmlTextReaderReadString

Return NULL if the node has no children or the children were already
deleted to match the 2.12 behavior.

Fixes #817.

Author: nwellnhof

xmlreader.c

@@ -1750,7 +1750,8 @@ xmlTextReaderReadString(xmlTextReaderPtr reader)
         case XML_CDATA_SECTION_NODE:
             break;
         case XML_ELEMENT_NODE:
-            if (xmlTextReaderDoExpand(reader) == -1)
+            if ((xmlTextReaderDoExpand(reader) == -1) ||
+                (node->children == NULL))
                 return(NULL);
             break;
         case XML_ATTRIBUTE_NODE: