main/distcc: fix segfault on aarch64

distcc segfaults on aarch64 systems:

  # distcc --fds
  Segmentation fault (core dumped)

Backport upstream patch fixing bad string length calculation in dcc_gcc_rewrite_fqn

Link: https://github.com/distcc/distcc/commit/879b71d6e95673e58d33f6c3c341a893ee307161

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master branch version (now merged): !29223 (merged)

Cc @nmeum (contributor) @ncopa (maintainer)