Promote the assertion that XLogBeginInsert() is not called twice into ERROR.

Seems like cheap insurance for WAL bugs. A spurious call to
XLogBeginInsert() in itself would be fairly harmless, but if there is any
data registered and the insertion is not completed/cancelled properly, there
is a risk that the data ends up in a wrong WAL record.

Per Jeff Janes's suggestion.

src/backend/access/transam/xloginsert.c

@@ -122,12 +122,14 @@ XLogBeginInsert(void)
 	Assert(max_registered_block_id == 0);
 	Assert(mainrdata_last == (XLogRecData *) &mainrdata_head);
 	Assert(mainrdata_len == 0);
-	Assert(!begininsert_called);
 
 	/* cross-check on whether we should be here or not */
 	if (!XLogInsertAllowed())
 		elog(ERROR, "cannot make new WAL entries during recovery");
 
+	if (begininsert_called)
+		elog(ERROR, "XLogBeginInsert was already called");
+
 	begininsert_called = true;
 }