short patch for bug #2184 'Prepared statements in embedded library was

broken with recent changes' (attempt 2).
Adding Statement_core is better because:
- set_statement() code is shorter and you don't need to modify it when adding
new members to Statement_core
- a bit faster (you don't have virtual call and don't free_root() twice)
Do that short patch instead in hope that set_statement() will be sooner or
later removed entirely

sql/sql_class.cc

@@ -336,6 +336,8 @@ THD::~THD()
 #ifndef DBUG_OFF
   dbug_sentry = THD_SENTRY_GONE;
 #endif  
+  /* Reset stmt_backup.mem_root to not double-free memory from thd.mem_root */
+  init_alloc_root(&stmt_backup.mem_root, 0, 0);
   DBUG_VOID_RETURN;
 }