Free intermediate buffers used by do-codec (CC #2068)

Leak from [CC #2068](http://curecode.org/rebol3/ticket.rsp?id=2068).  Found with valgrind but it would have been obvious even without.  Valgrind confirms no leaks afterward.

These two calls to Free_Mem seem to resolve the issue for now.  But it looks like the codec extensibility model is skeletal and ad-hoc at the moment.  See also this question:

http://stackoverflow.com/questions/14365034/how-do-you-write-a-codec-for-rebol-3

A general review of codecs that will work for letting people supply codecs written as Rebol code is needed.  That as well as native JPEG and GIF encoding (as opposed to just decoding)...choosing which codecs to include in minimal builds... among many other considerations.