Comment 19 for bug 1974214

I wonder if your build is failing to null-fill not-specifically-initialized
file-scope statics?

#0 0x00005620ff359503 in arc_sign (signspec=<optimized out>, sigheaders=0x5621001fc580, errstr=errstr@entry=0x7ffd081d0980) at ./b-exim4-daemon-custom/build-Linux-x86_64/arc.c:1663

   1660 if ((rheaders = arc_sign_scan_headers(&arc_sign_ctx, sigheaders)))
   1661 {
   1662 hdr_rlist ** rp;
   1663 for (rp = &headers_rlist; *rp; ) rp = &(*rp)->prev;
   1664 *rp = rheaders;
   1665 }

What do "p *rp" and "p headers_rlist" say for that core?
(On that theory, an "=NULL" at line 93 would help. But only for the arcsigning
case).