key-duplicates: Don't crash on redundant closing brackets or braces

Don't break on empty `context` stack when invalid YAML:

    [ a, b, c ] ]
    {a: 1, b: 2} }

tests/rules/test_key_duplicates.py

@@ -87,6 +87,10 @@ class KeyDuplicatesTestCase(RuleTestCase):
                    'anchor_reference:\n'
                    '  <<: *anchor_one\n'
                    '  <<: *anchor_two\n', conf)
+        self.check('---\n'
+                   '{a:1, b:2}}\n', conf, problem=(2, 11, 'syntax'))
+        self.check('---\n'
+                   '[a, b, c]]\n', conf, problem=(2, 10, 'syntax'))
 
     def test_enabled(self):
         conf = 'key-duplicates: enable'
@@ -165,6 +169,10 @@ class KeyDuplicatesTestCase(RuleTestCase):
                    'anchor_reference:\n'
                    '  <<: *anchor_one\n'
                    '  <<: *anchor_two\n', conf)
+        self.check('---\n'
+                   '{a:1, b:2}}\n', conf, problem=(2, 11, 'syntax'))
+        self.check('---\n'
+                   '[a, b, c]]\n', conf, problem=(2, 10, 'syntax'))
 
     def test_key_tokens_in_flow_sequences(self):
         conf = 'key-duplicates: enable'

yamllint/rules/key_duplicates.py

@@ -84,7 +84,8 @@ def check(conf, token, prev, next, nextnext, context):
     elif isinstance(token, (yaml.BlockEndToken,
                             yaml.FlowMappingEndToken,
                             yaml.FlowSequenceEndToken)):
-        context['stack'].pop()
+        if len(context['stack']) > 0:
+            context['stack'].pop()
     elif (isinstance(token, yaml.KeyToken) and
           isinstance(next, yaml.ScalarToken)):
         # This check is done because KeyTokens can be found inside flow