zlib: fix assert on bad input

The following test case occasionally triggered an assert because
write_in_progress_ didn't get cleared on error:

  $ cat test.js
  require('zlib').gunzip('BAM', console.log);
  setTimeout(gc, 10);

  $ while true; do node --expose-gc test.js || break; done
  { [Error: incorrect header check] errno: -3, code: 'Z_DATA_ERROR' }
  Assertion failed: (!write_in_progress_ && "write in progress"),
  function Clear, file ../src/node_zlib.cc, line 71.
  Abort trap: 6

Steps to avoid that:

* Initialize all primitive member fields in the constructor.
* Clear the write_in_progress_ member field in ZCtx::Error().
* Ref the ZCtx object as soon as write_in_progress_ is set to true.
  Before this commit, it could get GC'ed in the time between setting
  the field and the call to ctx->Ref().

Fixes #4783.