IMPORT: slz: fix header used for empty zlib message

IMPORT: slz: fix header used for empty zlib message

Calling slz_rfc1950_finish() without emitting any data would result in
incorrectly emitting a gzip header (rfc1952) instead of a zlib header
(rfc1950) due to a copy-paste between the two wrappers. The impact is
almost inexistent since the zlib format is almost never used in this
context, and compressing totally empty messages is quite rare as well.
Let's take this opportunity for fixing another mistake on an RFC number
in a comment.

This is slz upstream commit 7f3fce4f33e8c2f5e1051a32a6bca58e32d4f818.