zlib: Rename this variable to here (current decoding table entry)

There is no particular patch/description which described the reason for
this change but it was done as the part of zlib 1.2.3.5 release done by
zlib commit 639be997883d ("zlib 1.2.3.3") It is preparation for followup
patch.

Signed-off-by: Michal Simek <michal.simek@amd.com>