MDEV-15916 Change buffer crash during TRUNCATE or DROP TABLE

ibuf_restore_pos(): Do not issue any messages if the tablespace
is being dropped or truncated. In MariaDB 10.2, TRUNCATE TABLE
would not change the tablespace ID, and therefore the tablespace
would be found, even though TRUNCATE is in progress.

Furthermore, do not commit suicide if restoring the change buffer
cursor fails. The worst that could happen is that a secondary index
becomes corrupted due to incomplete change buffer merge.

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -4274,15 +4274,12 @@ ibuf_restore_pos(
 		return(TRUE);
 	}
 
-	if (fil_space_get_flags(space) == ULINT_UNDEFINED) {
-		/* The tablespace has been dropped.  It is possible
-		that another thread has deleted the insert buffer
-		entry.  Do not complain. */
-		ibuf_btr_pcur_commit_specify_mtr(pcur, mtr);
-	} else {
-		ib::error() << "ibuf cursor restoration fails!."
+	if (fil_space_t* s = fil_space_acquire_silent(space)) {
+		ib::error() << "ibuf cursor restoration fails!"
 			" ibuf record inserted to page "
-			<< space << ":" << page_no;
+			<< space << ":" << page_no
+			<< " in file " << s->chain.start->name;
+		fil_space_release(s);
 
 		ib::error() << BUG_REPORT_MSG;
 
@@ -4292,10 +4289,9 @@ ibuf_restore_pos(
 
 		rec_print_old(stderr,
 			      page_rec_get_next(btr_pcur_get_rec(pcur)));
-
-		ib::fatal() << "Failed to restore ibuf position.";
 	}
 
+	ibuf_btr_pcur_commit_specify_mtr(pcur, mtr);
 	return(FALSE);
 }