MDEV-80 Memory engine table full at much less than max_heap_table_size with btree index.

        RB-tree index in the MEMORY table fails if it grews over 4G.
        That happened because the old_allocated variable in hp_rb_write_key()
        had the uint type. Changed with the 'size_t' type to be same as the
        'rb_tree.allocated'.

per-file comments:
  storage/heap/hp_write.c
MDEV-80 Memory engine table full at much less than max_heap_table_size with btree index.
        uint->size_t for the 'old_allocated'.

storage/heap/hp_write.c

@@ -106,7 +106,7 @@ int hp_rb_write_key(HP_INFO *info, HP_KEYDEF *keyinfo, const uchar *record,
 		    uchar *recpos)
 {
   heap_rb_param custom_arg;
-  uint old_allocated;
+  size_t old_allocated;
 
   custom_arg.keyseg= keyinfo->seg;
   custom_arg.key_length= hp_rb_make_key(keyinfo, info->recbuf, record, recpos);