[SBE] Fix use-after-free bug with $arrayElemAt, $first, and $last

A bug picked up by the agg wildcard fuzzer here. This bug can crash the server or cause a BufBuilder to hit the 64 MB memory limit. The investigator should look into the interplay between doing an $arrayElemAt where the input array is computed from a $setUnion and the index is a NumberDecimal or simply -1. I observed the server crash when using the NumberDecimal and the -1 caused the BufBuilder to run out of memory.