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 4db3744.

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

Author: nathan-bossart

src/test/modules/test_shm_mq/worker.c

@@ -77,7 +77,7 @@ test_shm_mq_main(Datum main_arg)
 	 * exit, which is fine.  If there were a ResourceOwner, it would acquire
 	 * ownership of the mapping, but we have no need for that.
 	 */
-	seg = dsm_attach(DatumGetInt32(main_arg));
+	seg = dsm_attach(DatumGetUInt32(main_arg));
 	if (seg == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),