Be more careful about zlib return values

Be more careful about zlib return values

When creating a new object, we use "deflate(stream, Z_FINISH)" in a loop
until it no longer returns Z_OK, and then we do "deflateEnd()" to finish
up business.

That should all work, but the fact is, it's not how you're _supposed_ to
use the zlib return values properly:

 - deflate() should never return Z_OK in the first place, except if we
   need to increase the output buffer size (which we're not doing, and
   should never need to do, since we pre-allocated a buffer that is
   supposed to be able to hold the output in full). So the "while()" loop
   was incorrect: Z_OK doesn't actually mean "ok, continue", it means "ok,
   allocate more memory for me and continue"!

 - if we got an error return, we would consider it to be end-of-stream,
   but it could be some internal zlib error.  In short, we should check
   for Z_STREAM_END explicitly, since that's the only valid return value
   anyway for the Z_FINISH case.

 - we never checked deflateEnd() return codes at all.

Now, admittedly, none of these issues should ever happen, unless there is
some internal bug in zlib. So this patch should make zero difference, but
it seems to be the right thing to do.

We should probablybe anal and check the return value of "deflateInit()"
too!

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>