Fix failure when a shared tidbitmap has only one page.
Fix failure when a shared tidbitmap has only one page.

Commit 98e6e89040a0534ca26914c66cae9dd49ef62ad9 made inadequate
provision for the case of a single-page shared tidbitmap.  It
allocate space for a shared PagetableEntry, but failed to
initialize it.

Report by Thomas Munro.  Patch by Dilip Kumar, with some comment
changes by me.

Discussion: a97752045019d8433fa547b324cbda7d066568be@mail.gmail.com

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