Use correct DatumGet*() function in test_shm_mq_main().
This is purely cosmetic, as dsm_attach() interprets its argument as
a dsm_handle (i.e., an unsigned integer), but we might as well fix
it.
Oversight in commit
4db3744f1f.
Author: Jianghua Yang <yjhjstz@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAAZLFmRxkUD5jRs0W3K%3DUe4_ZS%2BRcAb0PCE1S0vVJBn3sWH2UQ%40mail.gmail.com
Backpatch-through: 13