Fabian Keil found the reason for a SIGBUS:
In the "gpg-agent --daemon" case, main() calls pth_kill()
after the client has been forked, so when es_deinit() is
called on exit, acquiring the estream_list_lock seems to
cause pth to dereference a pointer located in a memory
region that has previously been free()'d.
My approach to fix it is different than his suggestion. It should
allow to continue all estream operations after a pth_kill except for
restarting pth.
ce98524554