[libxslt] Fix quadratic runtime with text and <xsl:message>

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 4ccc06b56b8b6d39c29932c92cd1ed82f6698d6f
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sun Sep 20 15:14:47 2020 +0200

    Fix quadratic runtime with text and <xsl:message>
    
    Backup and restore "last text" data in xsltEvalTemplateString.
    Otherwise, optimization of string concatenation would be disabled
    whenever an xsl:message was processed.
    
    Found by OSS-Fuzz.

 libxslt/templates.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/libxslt/templates.c b/libxslt/templates.c
index 48b73a53..4108ed26 100644
--- a/libxslt/templates.c
+++ b/libxslt/templates.c
@@ -210,6 +210,8 @@ xsltEvalTemplateString(xsltTransformContextPtr ctxt,
 {
     xmlNodePtr oldInsert, insert = NULL;
     xmlChar *ret;
+    const xmlChar *oldLastText;
+    int oldLastTextSize, oldLastTextUse;
 
     if ((ctxt == NULL) || (contextNode == NULL) || (inst == NULL) ||
         (inst->type != XML_ELEMENT_NODE))
@@ -233,12 +235,18 @@ xsltEvalTemplateString(xsltTransformContextPtr ctxt,
     }
     oldInsert = ctxt->insert;
     ctxt->insert = insert;
+    oldLastText = ctxt->lasttext;
+    oldLastTextSize = ctxt->lasttsize;
+    oldLastTextUse = ctxt->lasttuse;
     /*
     * OPTIMIZE TODO: if inst->children consists only of text-nodes.
     */
     xsltApplyOneTemplate(ctxt, contextNode, inst->children, NULL, NULL);
 
     ctxt->insert = oldInsert;
+    ctxt->lasttext = oldLastText;
+    ctxt->lasttsize = oldLastTextSize;
+    ctxt->lasttuse = oldLastTextUse;
 
     ret = xmlNodeGetContent(insert);
     if (insert != NULL)