pgsql: Wrap multixact/members correctly during extension

Wrap multixact/members correctly during extension

In the 9.2 code for extending multixact/members, the logic was very
simple because the number of entries in a members page was a proper
divisor of 2^32, and thus at 2^32 wraparound the logic for page switch
was identical than at any other page boundary. In commit 0ac5ad5134f I
failed to realize this and introduced code that was not able to go over
the 2^32 boundary. Fix that by ensuring that when we reach the last
page of the last segment we correctly zero the initial page of the
initial segment, using correct uint32-wraparound-safe arithmetic.

Noticed while investigating bug #8673 reported by Serge Negodyuck, as
diagnosed by Andres Freund.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a50d97625497b76e3dc7c4aa22cd2c70317ec54d

Modified Files
--------------
src/backend/access/transam/multixact.c | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)