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.

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