MDEV-20213 binlog_encryption.binlog_incident failed in buildbot, server crashed in Check::validate

Check::validate(): Relax a debug assertion. TRX_SYS_SPACE fil_space_t
can be created and became visible to this assertion before
fil_system.sys_space becomes initialized

storage/innobase/fil/fil0fil.cc

@@ -4642,7 +4642,8 @@ struct	Check {
 		ut_list_validate(space->chain, check);
 		ut_a(space->size == check.size);
 		ut_ad(space->id != TRX_SYS_SPACE
-		      || space == fil_system.sys_space);
+		      || space == fil_system.sys_space
+		      || fil_system.sys_space == NULL);
 		ut_ad(space->id != SRV_TMP_SPACE_ID
 		      || space == fil_system.temp_space);
 		return(check.n_open);