tools/disktool: Properly tear down zlib stream
tools/disktool: Properly tear down zlib stream

- disktool was unconditionally calling deflateEnd() from fini(), even
when we weren't deflating.  Move zlib destruction to
{import,export}_image() and call the proper function.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr