[libxml2] Fix regression introduced with commit d88df4b

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 3fcf319378f9396a9ca840cd63b96a441818e1f1
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sat Aug 22 00:43:18 2020 +0200

    Fix regression introduced with commit d88df4b
    
    Revert the commit and use a different approach.
    
    Found by OSS-Fuzz.

 xinclude.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/xinclude.c b/xinclude.c
index c92b32bc8..f48e0af58 100644
--- a/xinclude.c
+++ b/xinclude.c
@@ -59,8 +59,8 @@ struct _xmlXIncludeRef {
     xmlNodePtr            inc; /* the included copy */
     int                   xml; /* xml or txt */
     int                 count; /* how many refs use that specific doc */
-    int                         skip; /* skip in case of errors */
     int                     fallback; /* fallback was loaded */
+    int                      emptyFb; /* flag to show fallback empty */
 };
 
 struct _xmlXIncludeCtxt {
@@ -1988,8 +1988,11 @@ xmlXIncludeLoadFallback(xmlXIncludeCtxtPtr ctxt, xmlNodePtr fallback, int nr) {
 
        ctxt->incTab[nr]->inc = xmlDocCopyNodeList(ctxt->doc,
                                                   fallback->children);
+        if (ctxt->incTab[nr]->inc == NULL)
+            ctxt->incTab[nr]->emptyFb = 1;
     } else {
         ctxt->incTab[nr]->inc = NULL;
+       ctxt->incTab[nr]->emptyFb = 1;  /* flag empty callback */
     }
     ctxt->incTab[nr]->fallback = 1;
     return(ret);
@@ -2153,7 +2156,6 @@ xmlXIncludeLoadNode(xmlXIncludeCtxtPtr ctxt, int nr) {
        }
     }
     if (ret < 0) {
-        ctxt->incTab[nr]->skip = 1;
        xmlXIncludeErr(ctxt, ctxt->incTab[nr]->ref,
                       XML_XINCLUDE_NO_FALLBACK,
                       "could not load %s, and no fallback was found\n",
@@ -2197,6 +2199,7 @@ xmlXIncludeIncludeNode(xmlXIncludeCtxtPtr ctxt, int nr) {
 
     list = ctxt->incTab[nr]->inc;
     ctxt->incTab[nr]->inc = NULL;
+    ctxt->incTab[nr]->emptyFb = 0;
 
     /*
      * Check against the risk of generating a multi-rooted document
@@ -2459,7 +2462,8 @@ xmlXIncludeDoProcess(xmlXIncludeCtxtPtr ctxt, xmlDocPtr doc, xmlNodePtr tree,
      *
      */
     for (i = ctxt->incBase;i < ctxt->incNr; i++) {
-       if (ctxt->incTab[i]->skip == 0)
+       if ((ctxt->incTab[i]->inc != NULL) ||
+           (ctxt->incTab[i]->emptyFb != 0))    /* (empty fallback) */
            xmlXIncludeIncludeNode(ctxt, i);
     }