libxslt: fix CVE-2019-18197

Added after 1.1.33 release.

CVE: CVE-2019-18197
Upstream-Status: Backport [https://gitlab.gnome.org/GNOME/libxslt.git]
Signed-off-by: Joe Slater <joe.slater@windriver.com>

commit 2232473733b7313d67de8836ea3b29eec6e8e285
Author: Nick Wellnhofer <wellnhofer@aevum.de>
Date:   Sat Aug 17 16:51:53 2019 +0200

    Fix dangling pointer in xsltCopyText
    
    xsltCopyText didn't reset ctxt->lasttext in some cases which could
    lead to various memory errors in relation with CDATA sections in input
    documents.
    
    Found by OSS-Fuzz.

diff --git a/libxslt/transform.c b/libxslt/transform.c
index 95ebd07..d7ab0b6 100644
--- a/libxslt/transform.c
+++ b/libxslt/transform.c
@@ -1094,6 +1094,8 @@ xsltCopyText(xsltTransformContextPtr ctxt, xmlNodePtr target,
 	    if ((copy->content = xmlStrdup(cur->content)) == NULL)
 		return NULL;
 	}
+
+	ctxt->lasttext = NULL;
     } else {
         /*
 	 * normal processing. keep counters to extend the text node