content_encoding: fix inflate_stream for no bytes available

- Don't call zlib's inflate() when avail_in stream bytes is 0.

This is a follow up to the parent commit 19e66e5. Prior to that change
libcurl's inflate_stream could call zlib's inflate even when no bytes
were available, causing inflate to return Z_BUF_ERROR, and then
inflate_stream would treat that as a hard error and return
CURLE_BAD_CONTENT_ENCODING.

According to the zlib FAQ, Z_BUF_ERROR is not fatal.

This bug would happen randomly since packet sizes are arbitrary. A test
of 10,000 transfers had 55 fail (ie 0.55%).

Ref: https://zlib.net/zlib_faq.html#faq05

Closes https://github.com/curl/curl/pull/2060

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