* fixes the zlib decompress sync flush bug as reported in bug #124981
  * avoids repeat calls to (in|de)flateEnd when destroying (de)compression
    objects
  * raises exception when allocating unused_data fails
  * fixes memory leak when allocating unused_data fails
  * raises exception when allocating decompress data fails
  * removes vestigial code from decompress flush now that decompression
    returns all available data
  * tidies code so object compress/decompress/flush routines are consistent