Gzip: fixed "zero size buf" alerts after ac5a741d39cf.

Gzip: fixed "zero size buf" alerts after ac5a741d39cf.

After ac5a741d39cf it is now possible that after zstream.avail_out
reaches 0 and we allocate additional buffer, there will be no more data
to put into this buffer, triggering "zero size buf" alert.  Fix is to
reset b->temporary flag in this case.

Additionally, an optimization added to avoid allocating additional buffer
in this case, by checking if last deflate() call returned Z_STREAM_END.
Note that checking for Z_STREAM_END by itself is not enough to fix alerts,
as deflate() can return Z_STREAM_END without producing any output if the
buffer is smaller than gzip trailer.

Reported by Witold Filipczyk,
http://mailman.nginx.org/pipermail/nginx-devel/2019-July/012469.html.