qcow2: use Z_OK instead of 0 for deflateInit2 return code check

Use appropriate macro, corresponding to deflateInit2 spec.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -3738,7 +3738,7 @@ static ssize_t qcow2_compress(void *dest, const void *src, size_t size)
     memset(&strm, 0, sizeof(strm));
     ret = deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED,
                        -12, 9, Z_DEFAULT_STRATEGY);
-    if (ret != 0) {
+    if (ret != Z_OK) {
         return -2;
     }