Avoid shifts of negative values inflateMark().

The C standard says that bit shifts of negative integers is
undefined.  This casts to unsigned values to assure a known
result.

(cherry picked from upstream github madler/zlib commit e54e1299404101a5a9d0cf5e45512b543967f958)

Bug: http://b/26133956
Change-Id: I1c641ca7d5802b2bdd7d1c266c3cc201bc5ae68a
1 file changed