libxml2: Patch for CVE-2018-14404

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901817 for more details.

diff --git a/xpath.c b/xpath.c
index f440696..87cfda2 100644
--- a/xpath.c
+++ b/xpath.c
@@ -13297,9 +13297,8 @@ xmlXPathCompOpEval(xmlXPathParserContextPtr ctxt, xmlXPathStepOpPtr op)
 		return(0);
 	    }
             xmlXPathBooleanFunction(ctxt, 1);
-            arg1 = valuePop(ctxt);
-            arg1->boolval &= arg2->boolval;
-            valuePush(ctxt, arg1);
+	    if (ctxt->value != NULL)
+	        ctxt->value->boolval &= arg2->boolval;
 	    xmlXPathReleaseObject(ctxt->context, arg2);
             return (total);
         case XPATH_OP_OR:
@@ -13323,9 +13322,8 @@ xmlXPathCompOpEval(xmlXPathParserContextPtr ctxt, xmlXPathStepOpPtr op)
 		return(0);
 	    }
             xmlXPathBooleanFunction(ctxt, 1);
-            arg1 = valuePop(ctxt);
-            arg1->boolval |= arg2->boolval;
-            valuePush(ctxt, arg1);
+	    if (ctxt->value != NULL)
+	        ctxt->value->boolval |= arg2->boolval;
 	    xmlXPathReleaseObject(ctxt->context, arg2);
             return (total);
         case XPATH_OP_EQUAL: