fktn-k / fkYAML / 31928315998

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Guard the context stack access behind a checked accessor (#552)

The deserializer reads m_context_stack.back() in a number of places where
the stack has not been proven non-empty at that point, which is undefined
behavior for a std::deque. Add a current_context() accessor which throws a
parse_error for an empty stack, and route those accesses through it.

The flow sequence/mapping beginning handlers are reachable with an empty
context stack: their default branch increments m_flow_context_depth without
pushing a parse context, so the matching flow ending can pop the stack empty
while the depth is still positive. The 7 byte inputs "{{{}},{" and "{{{}},["
crash even in an NDEBUG build before this change.

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