Fix possible race condition in Query cache.

sql/sql_cache.cc

@@ -1312,7 +1312,7 @@ ulong Query_cache::resize(ulong query_cache_size_arg)
     {
       BLOCK_LOCK_WR(block);
       Query_cache_query *query= block->query();
-      if (query && query->writer())
+      if (query->writer())
       {
         /*
            Drop the writer; this will cancel any attempts to store
@@ -1322,7 +1322,7 @@ ulong Query_cache::resize(ulong query_cache_size_arg)
         query->writer(0);
         refused++;
       }
-      BLOCK_UNLOCK_WR(block);
+      query->unlock_n_destroy();
       block= block->next;
     } while (block != queries_blocks);
   }
@@ -4247,11 +4247,11 @@ my_bool Query_cache::move_by_type(uchar **border,
     size_t key_length;
     key=query_cache_query_get_key((uchar*) block, &key_length, 0);
     my_hash_first(&queries, (uchar*) key, key_length, &record_idx);
+    block->query()->unlock_n_destroy();
+    block->destroy();
     // Move table of used tables
     memmove((char*) new_block->table(0), (char*) block->table(0),
 	   ALIGN_SIZE(n_tables*sizeof(Query_cache_block_table)));
-    block->query()->unlock_n_destroy();
-    block->destroy();
     new_block->init(len);
     new_block->type=Query_cache_block::QUERY;
     new_block->used=used;