The string with path to shell could be taken directly from struct passwd.

The string with path to shell could be taken directly from struct passwd.
At some point later the data it points to is overridden by getpwuid() call,
resulting in garbage. The problem could be easily demonstreated by double
doas call:

  $ doas doas -su _sndio
  doas: mpty: command not found

The fix is easy: just strdup() the pw_shell field value.

okay tedu@, tweaks from & okay natano@