util/deephash: fix test regression on 32-bit

Fix regression from bd93c3067e where I didn't notice the
32-bit test failure was real and not its usual slowness-related
regression. Yay failure blindness.

Updates #12526

Change-Id: I00e33bba697e2cdb61a0d76a71b62406f6c2eeb9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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