[libxml2] Fix dangling pointer with `xmllint --dropdtd`

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 72b3c067cedbb80dbbac755cca79ff502c858ad5
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Apr 22 19:24:50 2021 +0200

    Fix dangling pointer with `xmllint --dropdtd`
    
    Reset doc->intSubset when dropping the DTD.

 xmllint.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/xmllint.c b/xmllint.c
index dbef273a..a3fe10a2 100644
--- a/xmllint.c
+++ b/xmllint.c
@@ -2426,6 +2426,7 @@ static void parseAndPrintFile(char *filename, xmlParserCtxtPtr rectxt) {
        dtd = xmlGetIntSubset(doc);
        if (dtd != NULL) {
            xmlUnlinkNode((xmlNodePtr)dtd);
+            doc->intSubset = NULL;
            xmlFreeDtd(dtd);
        }
     }