Unset MyBEEntry, making elog.c's call to pgstat_get_my_query_id() safe.

Unset MyBEEntry, making elog.c's call to pgstat_get_my_query_id() safe.

Previously log messages late during shutdown could end up using either another
backend's PgBackendStatus (multi user) or segfault (single user) because
pgstat_get_my_query_id()'s check for !MyBEEntry didn't filter out use after
pgstat_beshutdown_hook().

This became a bug in 4f0b0966c86, but was a bit fishy before. But given
there's no known problematic cases before 14, it doesn't seem worth
backpatching further.

Also fixes a wrong filename in a comment, introduced in e1025044.

Reported-By: Andres Freund <andres@anarazel.de>
Reviewed-By: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/Julien Rouhaud <rjuju123@gmail.com>
Backpatch: 14-