xmllint: Fix UAF with --push --repeat

Short-lived regression. Fixes #841.

xmllint.c

diff --git a/xmllint.c b/xmllint.c
index 05b276f..272d8a5 100644
--- a/xmllint.c
+++ b/xmllint.c
@@ -349,6 +349,7 @@
         xmlParseChunk(ctxt, chars, 0, 1);
 
         doc = ctxt->myDoc;
+        ctxt->myDoc = NULL;
         if (f != stdin)
             fclose(f);
         return(doc);
@@ -427,6 +428,7 @@
         }
         htmlParseChunk(ctxt, chars, 0, 1);
         doc = ctxt->myDoc;
+        ctxt->myDoc = NULL;
         if (f != stdin)
             fclose(f);