Fixed memory leak introduces by a fix for MDEV-29932

The leaks are all 40 bytes and happens in this call stack when running
mtr vcol.vcol_syntax:

alloc_root()
...
Virtual_column_info::fix_and_check_exp()
...
Delayed_insert::get_local_table()

The problem was that one copied a MEM_ROOT from THD to a TABLE without
taking into account that new blocks would be allocated through the
TABLE memroot (and would thus be leaked).
In general, one should NEVER copy MEM_ROOT from one object to another
without clearing the copied memroot!

Fixed by, at end of get_local_table(), copy all new allocated objects
to client_thd->mem_root.

Other things:
- Removed references to MEM_ROOT::total_alloc that was wrongly left
  after a previous commit

include/my_sys.h

@@ -892,6 +892,7 @@ extern void init_alloc_root(MEM_ROOT *mem_root, const char *name,
 extern void *alloc_root(MEM_ROOT *mem_root, size_t Size);
 extern void *multi_alloc_root(MEM_ROOT *mem_root, ...);
 extern void free_root(MEM_ROOT *root, myf MyFLAGS);
+extern void move_root(MEM_ROOT *to, MEM_ROOT *from);
 extern void set_prealloc_root(MEM_ROOT *root, char *ptr);
 extern void reset_root_defaults(MEM_ROOT *mem_root, size_t block_size,
                                 size_t prealloc_size);

mysys/my_alloc.c

@@ -199,7 +199,6 @@ void *alloc_root(MEM_ROOT *mem_root, size_t length)
   next->left= 0;
   next->size= length;
   mem_root->used= next;
-  mem_root->total_alloc+= length;
   DBUG_PRINT("exit",("ptr: %p", (((char*) next)+
                                  ALIGN_SIZE(sizeof(USED_MEM)))));
   DBUG_RETURN((uchar*) (((char*) next)+ALIGN_SIZE(sizeof(USED_MEM))));
@@ -460,6 +459,28 @@ void set_prealloc_root(MEM_ROOT *root, char *ptr)
   }
 }
 
+/*
+  Move allocated objects from one root to another.
+
+  Notes:
+  We do not increase 'to->block_num' here as the variable isused to
+  increase block sizes in case of many allocations. This is special
+  case where this is not needed to take into account
+*/
+
+void move_root(MEM_ROOT *to, MEM_ROOT *from)
+{
+  USED_MEM *block, *next;
+  for (block= from->used; block ; block= next)
+  {
+    next= block->next;
+    block->next= to->used;
+    to->used= block;
+  }
+  from->used= 0;
+}
+
+
 
 char *strdup_root(MEM_ROOT *root, const char *str)
 {

sql/sql_insert.cc

@@ -2635,7 +2635,9 @@ TABLE *Delayed_insert::get_local_table(THD* client_thd)
   copy= new (copy_tmp) TABLE;
   *copy= *table;
   copy->vcol_refix_list.empty();
-  copy->mem_root= *client_thd->mem_root;
+  init_alloc_root(&copy->mem_root, client_thd->mem_root->name,
+                  client_thd->mem_root->block_size,
+                  0, MY_THREAD_SPECIFIC);
 
   /* We don't need to change the file handler here */
   /* Assign the pointers for the field pointers array and the record. */
@@ -2729,11 +2731,14 @@ TABLE *Delayed_insert::get_local_table(THD* client_thd)
   bzero((char*) bitmap, share->column_bitmap_size * bitmaps_used);
   copy->read_set=  &copy->def_read_set;
   copy->write_set= &copy->def_write_set;
+  move_root(client_thd->mem_root, &copy->mem_root);
+  free_root(&copy->mem_root, 0);
 
   DBUG_RETURN(copy);
 
   /* Got fatal error */
  error:
+  free_root(&copy->mem_root, 0);
   tables_in_use--;
   mysql_cond_signal(&cond);                     // Inform thread about abort
   DBUG_RETURN(0);