[PATCH] zlib merge: avoid 8-bit window errors

More merging from zlib-1.1.4

force windowBits > 8 to avoid a bug in the encoder for a window size
of 256 bytes. (A complete fix will be available in 1.1.5).

James Carlson:

        The problem is that s->strstart gets set to a very large
        positive integer when wsize (local copy of s->w_size) is
        subtracted in deflate.c:fill_window().  This happens because
        MAX_DIST(s) resolves as a negative number when the window size
        is 8 -- MAX_DIST(s) is defined as s->w_size-MIN_LOOKAHEAD in
        deflate.h.  MIN_LOOKAHEAD is MAX_MATCH+MIN_MATCH+1, and that
        is 258+3+1 or 262.  Since a window size of 8 gives s->w_size
        256, MAX_DIST(s) is 256-262 or -6.

        This results in read_buf() writing over memory outside of
        s->window, and a crash.

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