indentation: Don't crash on a closing flow token without an opening one

tests/rules/test_indentation.py

@@ -1089,6 +1089,25 @@ def test_broken_flows(self):
                    '  y, z\n'
                    ']\n', conf, problem=(3, 4))
 
+    def test_unmatched_flow_end(self):
+        # On invalid YAML, a closing flow token ('}' or ']') can appear with no
+        # matching opening one; PyYAML still emits the corresponding
+        # FlowMapping/FlowSequenceEndToken. yamllint must report the syntax
+        # error instead of crashing in the indentation rule.
+        # https://github.com/adrienverge/yamllint/issues/771
+        conf = ('indentation: {spaces: consistent}\n'
+                'document-start: disable\n')
+        self.check('data:\n'
+                   '  config.alloy: |\n'
+                   '\n'
+                   '  }\n', conf, problem=(4, 3, 'syntax'))
+        self.check('data:\n'
+                   '  config.alloy: |\n'
+                   '\n'
+                   '  ]\n', conf, problem=(4, 3, 'syntax'))
+        self.check('}\n', conf, problem=(1, 1, 'syntax'))
+        self.check(']\n', conf, problem=(1, 1, 'syntax'))
+
     def test_cleared_flows(self):
         # flow:
         #   [

yamllint/rules/indentation.py

@@ -339,7 +339,13 @@ def detect_indent(base_indent, next):
                 not isinstance(token, yaml.ValueToken)):
             expected = detect_indent(expected, token)
 
-        if found_indentation != expected:
+        # A closing flow token (']' or '}') is expected to align with the line
+        # that opened the collection, so `expected` is read from that opening
+        # token. On invalid YAML, PyYAML can emit a closing flow token with no
+        # matching opening token on the stack, leaving `expected` unset; skip
+        # the indentation check in that case and let the syntax error be
+        # reported instead of crashing.
+        if expected is not None and found_indentation != expected:
             if expected < 0:
                 message = (f'wrong indentation: expected at least '
                            f'{found_indentation + 1}')