Prevent access to other sessions' empty temp tables

Prevent access to other sessions' empty temp tables

Commit ce146621 ensures that ERROR is raised if a session tries to read
pages of another session's temp table.  But there is a corner case where
the other session's temp table is empty -- in this case the INSERT
command bypasses our checks and executes without any errors.

Such behavior is inconsistent and erroneous: it leaves an invalid buffer
in the temp buffers pool.  Since the buffer was created for another
session's temp table, we get an error "no such file or directory" when
trying to flush it.

This commit fixes it by adding a RELATION_IS_OTHER_TEMP check in the
relation-extension path.

Backpatch to 16, because it is the first release after 31966b151e6, which
introduced a separate local relation extension function
ExtendBufferedRelLocal(), which lacks of RELATION_IS_OTHER_TEMP() check.
As this fix introduces more checks to 013_temp_obj_multisession.pl, backpatch
the whole test script to 16.

Discussion: https://postgr.es/m/CAJDiXgiX2XZBHDNo%2BzBbvku%2BtchrUurvPRaN1_40mEQ1_sG90g%40mail.gmail.com
Author: Daniil Davydov <3danissimo@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Imran Zaheer <imran.zhir@gmail.com>
Reviewed-by: ZizhuanLiu X-MAN <44973863@qq.com>
Backpatch-through: 16