Bug on 32 bit systems?

Debian packaging ran into a problem with pandoc on i386, described at jgm/pandoc#9233. I think I've traced it to base64. Here is a Dockerfile that reproduces the segfault:

``` Docker
FROM i386/debian:sid
RUN echo 'deb-src http://deb.debian.org/debian/ unstable main' >> /etc/apt/sources.list
RUN apt update
RUN apt install -y cabal-install libghc-base64-dev libghc-bytestring-dev
RUN printf 'import "base64" Data.ByteString.Base64 (encodeBase64)\n\
import qualified Data.ByteString as B\n\
main = B.readFile "/usr/bin/whoami" >>= print . encodeBase64\n'\
> test.hs
RUN runghc -XPackageImports test.hs
```

Start by running the supplied i386/debian Dockerfile reproducer and reviewing the linked pandoc#9233 report. Trace the base64 call used by test.hs and investigate the 32-bit segfault; done means the reproducer completes without crashing on i386.

Written by the indexing model from the issue text.