From 3e77dd0c42dbc990e7d2e9b431846ff4ce9b4018 Mon Sep 17 00:00:00 2001
From: Leonardo de Moura <leonardo@microsoft.com>
Date: Wed, 11 Dec 2013 19:51:57 -0800
Subject: [PATCH] fix(kernel/context): make context remove more robust

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
---
 src/kernel/context.cpp | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/kernel/context.cpp b/src/kernel/context.cpp
index 13a2d2544..c8d7fe2f3 100644
--- a/src/kernel/context.cpp
+++ b/src/kernel/context.cpp
@@ -43,14 +43,18 @@ optional<context_entry> context::find(unsigned i) const {
 }
 
 static list<context_entry> remove_core(list<context_entry> const & l, unsigned s, unsigned n) {
-    if (s == 0) {
-        if (n > 0) {
-            return remove_core(tail(l), 0, n-1);
+    if (l) {
+        if (s == 0) {
+            if (n > 0) {
+                return remove_core(tail(l), 0, n-1);
+            } else {
+                return l;
+            }
         } else {
-            return l;
+            return cons(head(l), remove_core(tail(l), s-1, n));
         }
     } else {
-        return cons(head(l), remove_core(tail(l), s-1, n));
+        return l;
     }
 }