[libxml2] Fix xmlXPathIsNaN broken by recent commit

[Nick Wellnhofer <nwellnhof src gnome org>]

commit ddbb075b70abdc8166f878f039c5efd27a9df3e0
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Nov 27 14:30:19 2017 +0100

    Fix xmlXPathIsNaN broken by recent commit

 xpath.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/xpath.c b/xpath.c
index 5211b12..f440696 100644
--- a/xpath.c
+++ b/xpath.c
@@ -524,7 +524,7 @@ xmlXPathIsNaN(double val) {
 int
 xmlXPathIsInf(double val) {
 #ifdef isinf
-    return isinf(val);
+    return isinf(val) ? (val > 0 ? 1 : -1) : 0;
 #else
     if (val >= HUGE_VAL)
         return 1;