Bug#39494 : key_buffer_size > 4GB does not work on 64 bit Windows

Cache size  is truncated via 32bit ulong  in  ha_init_key_cache() and 
ha_resize_key_cache() 

This change fixes the cast to size_t instead of ulong. This cast is safe,
because key_buffer_size parameter is limited to SIZE_T_MAX

sql/handler.cc

@@ -3627,7 +3627,7 @@ int ha_init_key_cache(const char *name, KEY_CACHE *key_cache)
   if (!key_cache->key_cache_inited)
   {
     pthread_mutex_lock(&LOCK_global_system_variables);
-    ulong tmp_buff_size= (ulong) key_cache->param_buff_size;
+    size_t tmp_buff_size= (size_t) key_cache->param_buff_size;
     uint tmp_block_size= (uint) key_cache->param_block_size;
     uint division_limit= key_cache->param_division_limit;
     uint age_threshold=  key_cache->param_age_threshold;
@@ -3651,7 +3651,7 @@ int ha_resize_key_cache(KEY_CACHE *key_cache)
   if (key_cache->key_cache_inited)
   {
     pthread_mutex_lock(&LOCK_global_system_variables);
-    long tmp_buff_size= (long) key_cache->param_buff_size;
+    size_t tmp_buff_size= (size_t) key_cache->param_buff_size;
     long tmp_block_size= (long) key_cache->param_block_size;
     uint division_limit= key_cache->param_division_limit;
     uint age_threshold=  key_cache->param_age_threshold;