logical decoding: beware of an unset specinsert change

Coverity complains that there is no protection in the code (at least in
non-assertion-enabled builds) against speculative insertion failing to
follow the expected protocol.  Add an elog(ERROR) for the case.

src/backend/replication/logical/reorderbuffer.c

@@ -1519,6 +1519,8 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 					 * use as a normal record. It'll be cleaned up at the end
 					 * of INSERT processing.
 					 */
+					if (specinsert == NULL)
+						elog(ERROR, "invalid ordering of speculative insertion changes");
 					Assert(specinsert->data.tp.oldtuple == NULL);
 					change = specinsert;
 					change->action = REORDER_BUFFER_CHANGE_INSERT;