MDEV-22751 Uninitialized tbl_len in dict_acquire_mdl_shared()

A crash was observed where dict_acquire_mdl_shared<trylock=false>
would invoke memcpy() with an apparently uninitialized tbl_len.

dict_table_t::parse_name(): Remove an unnecessary tbl_len--
operation. (This should be mostly non-functional cleanup.)

dict_acquire_mdl_shared(): If the second dict_table_t::parse_name()
returns false, terminate the loop just like we would do on the
first invocation.

storage/innobase/dict/dict0dict.cc

@@ -749,7 +749,6 @@ bool dict_table_t::parse_name(char (&db_name)[NAME_LEN + 1],
 
   size_t tbl_len= strlen(name.m_name + db_len);
   memcpy(tbl_buf, name.m_name + db_len + 1, tbl_len);
-  tbl_len--;
   if (!dict_locked)
     mutex_exit(&dict_sys.mutex);
 
@@ -883,7 +882,17 @@ dict_acquire_mdl_shared(dict_table_t *table,
 
   size_t db1_len, tbl1_len;
 
-  table->parse_name<!trylock>(db_buf1, tbl_buf1, &db1_len, &tbl1_len);
+  if (!table->parse_name<!trylock>(db_buf1, tbl_buf1, &db1_len, &tbl1_len))
+  {
+    /* The table was renamed to #sql prefix.
+    Release MDL (if any) for the old name and return. */
+    if (*mdl)
+    {
+      mdl_context->release_lock(*mdl);
+      *mdl= nullptr;
+    }
+    return table;
+  }
 
   if (*mdl)
   {