how I can get to the coffgen.c file

It's part of the binutils package I linked to above (21MB tarball).

Anyhow, in the meantime I had already taken a look at that line. The assertion reads:

    BFD_ASSERT (sym->_n._n_n._n_offset >= STRING_SIZE_SIZE);
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Unforunately, the reason the assertion fails seems to be just another indication of the same problem which is being reported later as

   ...\libmsvcrt.a(diwcs00633.o): local symbol `     ' has no section
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(At least that's my guess.)  I have no hypothesis where this strange empty '     ' symbol comes from, and I can't replicate the issue here.  I'm sorry I can't help any further.