[flaky tests] Address SIGSEGV on ResultTracker while running alter_table_randomized-test
[flaky tests] Address SIGSEGV on ResultTracker while running alter_table_randomized-test

This addresses a flakyness in the alter_table_randomized-test whereby
calling ResultTracker::FailAndRespond() on a transaction that was never
tracked (a legal and possible thing for follower transactions) would
cause a SIGSEGV. The SIGSEGV is caused because we try to deference
the CompletionRecord* to track its memory, even though we already
know it doesn't exist.

This is hard to reproduce in either exactly once tests, but the change
is pretty obvious.

I also added some comment on how that can happen, it took me a while
to find the path that could cause this (again).

Change-Id: I961af334fa2dd7faff0e95c7a49f2f16b2096fe0
Reviewed-on: http://gerrit.cloudera.org:8080/4629
Reviewed-by: Dan Burkert <2591e5f46f28d303f9dc027d475a5c60d8dea17a@cloudera.com>
Tested-by: David Ribeiro Alves <78b9f953b197533e9b51c860b080869056433b48@apache.org>
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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