add missing begin/ensure block

Author: pitr-ch

lib/concurrent/synchronization/rbx_lockable_object.rb

@@ -20,9 +20,12 @@ def synchronize(&block)
         else
           result = nil
           Rubinius.synchronize(self) do
-            @__owner__ = Thread.current
-            result     = yield
-            @__owner__ = nil
+            begin
+              @__owner__ = Thread.current
+              result     = yield
+            ensure
+              @__owner__ = nil
+            end
           end
           result
         end