Fix a strict aliasing violation

This one is almost a textbook example of an aliasing violation, and it
is straightforward to fix, so clean it up.  (The warning only shows up
if you remove the -fno-strict-aliasing option.)  Also, move the code
after the error checking.  Doesn't make a difference technically, but
it seems strange to do actions before errors are checked.

Reported-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/20240724.155525.366150353176322967.ishii%40postgresql.org

src/backend/replication/logical/origin.c

@@ -789,14 +789,6 @@ StartupReplicationOrigin(void)
 
 		readBytes = read(fd, &disk_state, sizeof(disk_state));
 
-		/* no further data */
-		if (readBytes == sizeof(crc))
-		{
-			/* not pretty, but simple ... */
-			file_crc = *(pg_crc32c *) &disk_state;
-			break;
-		}
-
 		if (readBytes < 0)
 		{
 			ereport(PANIC,
@@ -805,6 +797,13 @@ StartupReplicationOrigin(void)
 							path)));
 		}
 
+		/* no further data */
+		if (readBytes == sizeof(crc))
+		{
+			memcpy(&file_crc, &disk_state, sizeof(file_crc));
+			break;
+		}
+
 		if (readBytes != sizeof(disk_state))
 		{
 			ereport(PANIC,