MDEV-33622 Server crashes when the UPDATE statement (which has duplicate key)...

MDEV-33622 Server crashes when the UPDATE statement (which has duplicate key) is run after setting a low thread_stack

This was caused by wrong allocation of variable on stack.
(Was allocating 4K of data instead of 512 bytes).

No test case as the original MDEV test cases is not usable for mtr.

include/my_bitmap.h

@@ -88,6 +88,7 @@ extern void bitmap_lock_clear_bit(MY_BITMAP *map, uint bitmap_bit);
 
 #define my_bitmap_map_bytes sizeof(my_bitmap_map)
 #define my_bitmap_map_bits  (my_bitmap_map_bytes*8)
+/* Size in bytes to store 'bits' number of bits */
 #define bitmap_buffer_size(bits) (MY_ALIGN((bits), my_bitmap_map_bits)/8)
 #define my_bitmap_buffer_size(map) bitmap_buffer_size((map)->n_bits)
 #define no_bytes_in_export_map(map) (((map)->n_bits + 7)/8)

sql/sql_update.cc

@@ -257,7 +257,7 @@ static void prepare_record_for_error_message(int error, TABLE *table)
   Field *field;
   uint keynr;
   MY_BITMAP unique_map; /* Fields in offended unique. */
-  my_bitmap_map unique_map_buf[bitmap_buffer_size(MAX_FIELDS)];
+  my_bitmap_map unique_map_buf[bitmap_buffer_size(MAX_FIELDS)/sizeof(my_bitmap_map)];
   DBUG_ENTER("prepare_record_for_error_message");
 
   /*