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.

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