pgsql: Fix broken Assert() introduced by 8e9a16ab8f7f0e58

Fix broken Assert() introduced by 8e9a16ab8f7f0e58

Don't assert MultiXactIdIsRunning if the multi came from a tuple that
had been share-locked and later copied over to the new cluster by
pg_upgrade.  Doing that causes an error to be raised unnecessarily:
MultiXactIdIsRunning is not open to the possibility that its argument
came from a pg_upgraded tuple, and all its other callers are already
checking; but such multis cannot, obviously, have transactions still
running, so the assert is pointless.

Noticed while investigating the bogus pg_multixact/offsets/0000 file
left over by pg_upgrade, as reported by Andres Freund in
http://www.postgresql.org/message-id/20140530121631.GE25431@alap3.anarazel.de

Backpatch to 9.3, as the commit that introduced the buglet.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5c538442a0a73b9d4f3fa11ecac6d5b93a982aca

Modified Files
--------------
src/backend/access/heap/heapam.c |    8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)