[libxslt] Avoid recursion in keys.c:skipPredicate

[Nick Wellnhofer <nwellnhof src gnome org>]

commit d8ce4f1c27cdcbe0b202a696d636d2122abb192e
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sun Sep 15 12:15:08 2019 +0200

    Avoid recursion in keys.c:skipPredicate
    
    Fixes potential call stack overflow found by OSS-Fuzz.

 libxslt/keys.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)
---
diff --git a/libxslt/keys.c b/libxslt/keys.c
index a1f150aa..ecef5382 100644
--- a/libxslt/keys.c
+++ b/libxslt/keys.c
@@ -241,6 +241,8 @@ skipString(const xmlChar *cur, int end) {
  */
 static int
 skipPredicate(const xmlChar *cur, int end) {
+    int level = 0;
+
     if ((cur == NULL) || (end < 0)) return(-1);
     if (cur[end] != '[') return(end);
     end++;
@@ -251,12 +253,12 @@ skipPredicate(const xmlChar *cur, int end) {
                return(-1);
            continue;
        } else if (cur[end] == '[') {
-           end = skipPredicate(cur, end);
-           if (end <= 0)
-               return(-1);
-           continue;
-       } else if (cur[end] == ']')
-           return(end + 1);
+            level += 1;
+       } else if (cur[end] == ']') {
+            if (level == 0)
+               return(end + 1);
+            level -= 1;
+        }
        end++;
     }
     return(-1);