Zero padding in replication origin's checkpointed on disk-state.

This seems to be largely cosmetic, avoiding valgrind bleats and the
like. The uninitialized padding influences the CRC of the on-disk
entry, but because it's also used when verifying the CRC, that doesn't
cause spurious failures.  Backpatch nonetheless.

It's a bit unfortunate that contrib/test_decoding/sql/replorigin.sql
doesn't exercise the checkpoint path, but checkpoints are fairly
expensive on weaker machines, and we'd have to stop/start for that to
be meaningful.

Author: Andres Freund
Discussion: https://postgr.es/m/20170422183123.w2jgiuxtts7qrqaq@alap3.anarazel.de
Backpatch: 9.5, where replication origins were introduced

src/backend/replication/logical/origin.c

@@ -559,6 +559,9 @@ CheckPointReplicationOrigin(void)
 		if (curstate->roident == InvalidRepOriginId)
 			continue;
 
+		/* zero, to avoid uninitialized padding bytes */
+		memset(&disk_state, 0, sizeof(disk_state));
+
 		LWLockAcquire(&curstate->lock, LW_SHARED);
 
 		disk_state.roident = curstate->roident;