[Backport #3567] r28082 * ext/zlib/zlib.c (zstream_append_input2): add RB_GC_GUARD

Backport #3567: r28082 * ext/zlib/zlib.c (zstream_append_input2): add
RB_GC_GUARD.
http://redmine.ruby-lang.org/issues/show/3567

e$B5/I<<Te(B: Nobuyoshi N.
e$B%9%F!<%?%9e(B: Assigned, e$BM%@hEYe(B: Normal
e$BC4Ev<Te(B: Yuki S., e$B%+%F%4%je(B: ext

1.9.1e$B$K$bI,MW$@$H;W$$$^$9!#e(B