Suppress compiler warning in slab.c.
Suppress compiler warning in slab.c.

Compilers that don't realize that elog(ERROR) doesn't return
complained that SlabRealloc() failed to return a value.

While at it, fix the rather muddled header comment for the function.

Per buildfarm.

greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb