rust: fix possible out-of-bounds read through index_get_parents()
index_get_parents() is an internal function, which doesn't check if the
specified rev is valid. If rustlazyancestors() were instantiated with an
invalid stoprev, it would access to invalid memory region.
This is NOT a security fix as there's no Python code triggering the bug,
but included in this series to not give a notion about the memory issue
fixed by the previous patch.
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