Fix zlib-error-message to include human-readable version

The error string is looked up in an array, but because zlib
error codes are negative, the error's string was always "f".

basis/compression/zlib/zlib-tests.factor

@@ -1,9 +1,12 @@
 ! Copyright (C) 2009 Doug Coleman.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: kernel tools.test compression.zlib classes ;
+USING: accessors kernel tools.test compression.zlib classes ;
+QUALIFIED-WITH: compression.zlib.ffi ffi
 IN: compression.zlib.tests
 
 : compress-me ( -- byte-array ) B{ 1 2 3 4 5 } ;
 
 [ t ] [ compress-me [ compress uncompress ] keep = ] unit-test
 [ t ] [ compress-me compress compressed instance? ] unit-test
+
+[ ffi:Z_DATA_ERROR zlib-error-message ] [ string>> "data error" = ] must-fail-with

basis/compression/zlib/zlib.factor

@@ -19,7 +19,9 @@ ERROR: zlib-failed n string ;
     dup compression.zlib.ffi:Z_ERRNO = [
         drop errno "native libc error"
     ] [
-        dup {
+        dup
+        -1 * ! zlib error codes are negative
+        {
             "no error" "libc_error"
             "stream error" "data error"
             "memory error" "buffer error" "zlib version error"