xinclude: Don't raise error on empty nodeset

xmlXPtrEval returning NULL means an empty nodeset if no error code is
set.

See #733.

Author: nwellnhof

xinclude.c

@@ -1471,7 +1471,7 @@ xmlXIncludeLoadDoc(xmlXIncludeCtxtPtr ctxt, xmlXIncludeRefPtr ref) {
 #endif
         }
 	xptr = xmlXPtrEval(fragment, ctxt->xpctxt);
-	if (xptr == NULL) {
+	if (ctxt->xpctxt->lastError.code != XML_ERR_OK) {
             if (ctxt->xpctxt->lastError.code == XML_ERR_NO_MEMORY)
                 xmlXIncludeErrMemory(ctxt);
             else
@@ -1480,6 +1480,8 @@ xmlXIncludeLoadDoc(xmlXIncludeCtxtPtr ctxt, xmlXIncludeRefPtr ref) {
                                fragment);
             goto error;
 	}
+        if (xptr == NULL)
+            goto done;
 	switch (xptr->type) {
 	    case XPATH_UNDEFINED:
 	    case XPATH_BOOLEAN:
@@ -1559,6 +1561,7 @@ xmlXIncludeLoadDoc(xmlXIncludeCtxtPtr ctxt, xmlXIncludeRefPtr ref) {
     }
 #endif
 
+done:
     ret = 0;
 
 error: