zlib.c: check EOF

* ext/zlib/zlib.c (gzfile_read, gzfile_read_all, gzfile_getc),
  (gzreader_gets): check EOF.  [ruby-core:55220] [Bug #8467]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41014 b2dd03c8-39d4-4d8f-98ff-823fe69b080e