The iteration code used `g_string_overwrite_len()` to try and simplify
buffer allocation and growth, but seemingly forgot to handle the fact
that it doesn’t nul-terminate what it overwrites: the method is intended
to be used to splice bits into longer strings, not to overwrite an
entire nul-terminated string.

This meant that when iterating over a comma-separated `c_prefix` like
`GUnix,G`, on the second iteration `g_string_overwrite_len()` would be
used to write `G` into index 0 of the already-set `GUnix` string in the
buffer, leading to the first iteration happening all over again and the
`G` prefix being ignored.

This led to symbols failing to be matched to the `GioUnix` typelib, even
though they should have been.

This will be checked by a test in the following commit.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3303