zlib: Add initialization to satisfy valgrind

Restore the change from commit cf133ff6b3 (Fix uninitialized variable
access in zlib, 2009-11-18, v2.8.2~705). Their web site claims it does
no harm ( https://www.zlib.net/zlib_faq.html#faq36), but fixing it this
way eliminates the diagnostic.
1 file changed