Right now, different subclasses of `MatchExpression` have different failure behavior for the `getChild(size_t index)` function. Some have an invariant, while some will blindly pass on the out-of-bounds index to the underlying vector, which will result in a segfault. This behavior should be standardized, either with invariant (what some do currently) or tassert (probably makes more sense, no reason to crash the whole server in this case).