pgsql: Fix bug that could try to freeze running multixacts.

Fix bug that could try to freeze running multixacts.

Commits 801c2dc7 and 801c2dc7 made it possible for vacuum to
try to freeze a multixact that is still running.  That was
prevented by a check, but raised an error.  Repair.

Back-patch all the way.

Author: Nathan Bossart, Jeremy Schneider
Reported-by: Jeremy Schneider
Reviewed-by: Jim Nasby, Thomas Munro
Discussion: https://postgr.es/m/DAFB8AFF-2F05-4E33-AD7F-FF8B0F760C17%40amazon.com

Branch
------
REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6f1e336de01003e4005a9c00f6c25c94ccd4dd34

Modified Files
--------------
src/backend/commands/vacuum.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)