pgsql: Allow HotStandbyActiveInReplay() to be called in single user mod

Allow HotStandbyActiveInReplay() to be called in single user mode.

HotStandbyActiveInReplay, introduced in 061b079f, only allowed WAL
replay to happen in the startup process, missing the single user case.

This buglet is fairly harmless as it only causes problems when single
user mode in an assertion enabled build is used to replay a btree vacuum
record.

Backpatch to 9.2. 061b079f was backpatched further, but the assertion
was not.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/18935145e7f31ca975e0763f73c8c3f12aa62672

Modified Files
--------------
src/backend/access/transam/xlog.c |    2 +-
1 file changed, 1 insertion(+), 1 deletion(-)