move call to xmlKeepBlanksDefault(0) to before the creation of an XML parser context

the value is used by the parser context; the old code called it only after the *first* parser context
was created. therefore the first XMLTree::read() call has its behavior determined by libxml2's default
for this value, rather than by our explicit choice (do not treat whitespace as a note). All subequent
read() calls will use our value.

This variable inside libxml2 is actually per-thread, which just increases the stakes for calling
xmlKeepBlanksDefault() at the right time

libs/pbd/xml++.cc

@@ -85,13 +85,17 @@ XMLTree::read_internal(bool validate)
 		_doc = 0;
 	}
 
+	/* Calling this prevents libxml2 from treating whitespace as active
+	   nodes. It needs to be called before we create a parser context.
+	*/
+	xmlKeepBlanksDefault(0);
+
 	/* create a parser context */
 	xmlParserCtxtPtr ctxt = xmlNewParserCtxt();
 	if (ctxt == NULL) {
 		return false;
 	}
 
-	xmlKeepBlanksDefault(0);
 	/* parse the file, activating the DTD validation option */
 	if (validate) {
 		_doc = xmlCtxtReadFile(ctxt, _filename.c_str(), NULL, XML_PARSE_DTDVALID);