Initialize index stats during parallel VACUUM.
Initialize shared memory allocated for index stats to avoid a hard
crash. This was possible when parallel VACUUM became confused about the
current phase of index processing.
Oversight in commit
8e1fae1938, which refactored parallel VACUUM.
Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reported-By: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/
20220818133406.GL26426@telsasoft.com
Backpatch: 15-, the first version with the refactoring commit.