MDEV-18496 Crash when Aria encryption is enabled but plugin not available

fix uninitialized struct member

mysql-test/suite/maria/encrypt-no-key.result

@@ -5,5 +5,11 @@ alter table t1 disable keys;
 insert into t1 values  (1,1);
 alter table t1 enable keys;
 ERROR HY000: Unknown key id 1. Can't continue!
+repair table t1 use_frm;
+Table	Op	Msg_type	Msg_text
+test.t1	repair	warning	Number of rows changed from 0 to 1
+test.t1	repair	Error	Unknown key id 1. Can't continue!
+test.t1	repair	Error	Unknown key id 1. Can't continue!
+test.t1	repair	status	OK
 drop table t1;
 set global aria_encrypt_tables= default;

mysql-test/suite/maria/encrypt-no-key.test

@@ -9,5 +9,6 @@ alter table t1 disable keys;
 insert into t1 values  (1,1);
 error 192;
 alter table t1 enable keys;
+repair table t1 use_frm;
 drop table t1;
 set global aria_encrypt_tables= default;

storage/maria/ma_check.c

@@ -3198,6 +3198,7 @@ static int write_page(MARIA_SHARE *share, File file,
   args.page= buff;
   args.pageno= (pgcache_page_no_t) (pos / share->block_size);
   args.data= (uchar*) share;
+  args.crypt_buf= NULL;
   (* share->kfile.pre_write_hook)(&args);
   res= my_pwrite(file, args.page, block_size, pos, myf_rw);
   (* share->kfile.post_write_hook)(res, &args);