MDEV-11646 main.myisam, maria.maria, main.mix2_myisam, main.myisampack, main.mrr_icp_extra fail in buildbot with valgrind (Syscall param pwrite64(buf) points to uninitialised byte(s))

If the table has a varchar column and a forced fixed for format
(as in varchar.inc), Field_varstring::store() will only store the
actual number of bytes, not padded, in the record[0].

That is, on inserts a part of record[0] can be uninitialized.

Fix: initialize record[0] when a TABLE is created, it doesn't matter
what kind of garbage can be in this unused/invisible part of the
record, as long as it's not some random memory contents
(that can contain sensitive data).

sql/sql_insert.cc

@@ -970,11 +970,6 @@ bool mysql_insert(THD *thd,TABLE_LIST *table_list,
           be overwritten by fill_record() anyway (and fill_record() does not
           use default values in this case).
         */
-#ifdef HAVE_valgrind
-        if (table->file->ha_table_flags() && HA_RECORD_MUST_BE_CLEAN_ON_WRITE)
-          restore_record(table,s->default_values);	// Get empty record
-        else
-#endif
         table->record[0][0]= share->default_values[0];
 
         /* Fix undefined null_bits. */

sql/table.cc

@@ -4150,16 +4150,7 @@ void TABLE::init(THD *thd, TABLE_LIST *tl)
 
   DBUG_ASSERT(key_read == 0);
 
-  /* mark the record[0] uninitialized */
-  TRASH(record[0], s->reclength);
-
-  /*
-    Initialize the null marker bits, to ensure that if we are doing a read
-    of only selected columns (like in keyread), all null markers are
-    initialized.
-  */
-  memset(record[0], 255, s->null_bytes); 
-  memset(record[1], 255, s->null_bytes); 
+  restore_record(this, s->default_values);
 
   /* Tables may be reused in a sub statement. */
   DBUG_ASSERT(!file->extra(HA_EXTRA_IS_ATTACHED_CHILDREN));