Only initialize WAL shmem once in EXEC_BACKEND builds

This only affects EXEC_BACKEND/Windows builds which we currently do not
support, but we fix this anyway to make the code more consistent and
easier to understand since we try to care about this in other places.

In the future we may want to add CI and proper support for EXEC_BACKEND
builds.

The issue was originally found by Zsolt Parragi.

Author: jeltz

contrib/pg_tde/src/access/pg_tde_xlog_smgr.c

@@ -160,25 +160,28 @@ TDEXLogEncryptStateSize(void)
 void
 TDEXLogShmemInit(void)
 {
-	bool		foundBuf;
+	bool		found;
 
 	EncryptionState = (EncryptionStateData *)
 		ShmemInitStruct("TDE XLog Encryption State",
 						TDEXLogEncryptStateSize(),
-						&foundBuf);
+						&found);
 
-	memset(EncryptionState, 0, sizeof(EncryptionStateData));
+	if (!found)
+	{
+		memset(EncryptionState, 0, sizeof(EncryptionStateData));
+
+		pg_atomic_init_u64(&EncryptionState->enc_key_lsn, 0);
+
+		elog(DEBUG1, "pg_tde: initialized encryption buffer %lu bytes", TDEXLogEncryptStateSize());
+	}
 
 	if (EncryptXLog)
 	{
 		EncryptionBuf = (char *) TYPEALIGN(PG_IO_ALIGN_SIZE, ((char *) EncryptionState) + sizeof(EncryptionStateData));
 
 		Assert((char *) EncryptionState + TDEXLogEncryptStateSize() >= (char *) EncryptionBuf + TDEXLogEncryptBuffSize());
 	}
-
-	pg_atomic_init_u64(&EncryptionState->enc_key_lsn, 0);
-
-	elog(DEBUG1, "pg_tde: initialized encryption buffer %lu bytes", TDEXLogEncryptStateSize());
 }
 
 #else							/* !FRONTEND */