pgsql: Modify ShmemInitStruct and ShmemInitHash to throw errors

Log Message:
-----------
Modify ShmemInitStruct and ShmemInitHash to throw errors internally,
rather than returning NULL for some-but-not-all failures as they used to.
Remove now-redundant tests for NULL from call sites.

We had to do something about this because many call sites were failing to
check for NULL; and changing it like this seems a lot more useful and
mistake-proof than adding checks to the call sites without them.

Modified Files:
--------------
    pgsql/contrib/pg_stat_statements:
        pg_stat_statements.c (r1.13 -> r1.14)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_stat_statements/pg_stat_statements.c?r1=1.13&r2=1.14)
    pgsql/doc/src/sgml:
        xfunc.sgml (r1.145 -> r1.146)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml?r1=1.145&r2=1.146)
    pgsql/src/backend/access/transam:
        slru.c (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/slru.c?r1=1.49&r2=1.50)
    pgsql/src/backend/commands:
        async.c (r1.156 -> r1.157)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c?r1=1.156&r2=1.157)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.109 -> r1.110)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.109&r2=1.110)
        bgwriter.c (r1.67 -> r1.68)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.67&r2=1.68)
    pgsql/src/backend/replication:
        walreceiverfuncs.c (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walreceiverfuncs.c?r1=1.4&r2=1.5)
        walsender.c (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.18&r2=1.19)
    pgsql/src/backend/storage/buffer:
        buf_table.c (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_table.c?r1=1.51&r2=1.52)
    pgsql/src/backend/storage/ipc:
        shmem.c (r1.103 -> r1.104)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/shmem.c?r1=1.103&r2=1.104)
    pgsql/src/backend/storage/lmgr:
        lock.c (r1.196 -> r1.197)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c?r1=1.196&r2=1.197)
        proc.c (r1.217 -> r1.218)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c?r1=1.217&r2=1.218)