Always initialize @default and @default_block for AbstractThreadLocalVar

This avoids Ruby warnings like this when accessing the default value:

> lib/concurrent/atomic/abstract_thread_local_var.rb:57: warning: instance variable @default_block not initialized

Author: meineerde

lib/concurrent/atomic/abstract_thread_local_var.rb

@@ -15,7 +15,9 @@ def initialize(default = nil, &default_block)
 
       if block_given?
         @default_block = default_block
+        @default = nil
       else
+        @default_block = nil
         @default = default
       end