xsltAddTextString: Check for overflow when merging text nodes.

third_party/libxslt/libxslt/xsltInternals.h

Index: third_party/libxslt/libxslt/xsltInternals.h
diff --git a/third_party/libxslt/libxslt/xsltInternals.h b/third_party/libxslt/libxslt/xsltInternals.h
index 060b17837cac5a2e529a323dcf6628c768541ffe..5ad177196c5ca92322a8c58b27185d7547f00ce2 100644
--- a/third_party/libxslt/libxslt/xsltInternals.h
+++ b/third_party/libxslt/libxslt/xsltInternals.h
@@ -1754,8 +1754,8 @@ struct _xsltTransformContext {
      * Speed optimization when coalescing text nodes
      */
     const xmlChar  *lasttext;		/* last text node content */
-    unsigned int    lasttsize;		/* last text node size */
-    unsigned int    lasttuse;		/* last text node use */
+    int             lasttsize;		/* last text node size */
+    int             lasttuse;		/* last text node use */
     /*
      * Per Context Debugging
      */