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

I've also asked upstream if they'd like to cut a new release for this fix that dates quite a bit:
https://github.com/distcc/distcc/issues/450
but I believe stable 3.15 would be better off with just this backport, so it's worth merging on its own right for backporting purposes. I can open a follow up MR for 3.15 once this gets merged.

Cc @nmeum (contributor) @ncopa (maintainer) @Leo @omni (recent update commits)