Guard access to analysis results

[frankmcsherry]

The Analysis framework reveals the results of expression children as part of the ongoing derivation, but this means that information about non-children may not be available, and in particular information about future bindings in a LetRec are not available. This means that implementors must themselves guard against out-of-bounds access based on these identifiers, using e.g. results.get(id) rather than results[id].

There seemed to be only one instance of this, in our cardinality computation. It seems localized to Get expression handling.

fixes MaterializeInc/database-issues#8079

Motivation

Tips for reviewer

Checklist

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[ggevay]

Thanks for the fix!

Would be great to add @mgree's repro in cardinality.slt. (Possibly with not checking the EXPLAIN output, just with statement ok.)

[frankmcsherry]

Thanks for the quick review!