zlib: check callback is function

[jasnell]

joyent/node PR nodejs/node-v0.x-archive#6525 was an attempt to add
checking to zlib to ensure that the callback was a function. That PR cannot land
for a variety of reasons. I note, however, that zlib, and other parts of the code still
do not check callbacks consistently. I know this has been discussed a number of
times, but since I'm closing the original PR, I wanted to make sure the issue wasn't
lost.

[Fishrock123]

#2596 (comment)

Closing, the error here denotes a bug in user code. There is no way to get the resulting data otherwise, and it is a no-op.

[kulkarniankita]

Is this still a issue and needs to be fixed?