trailer: handle NULL value when parsing trailer-specific config

trailer: handle NULL value when parsing trailer-specific config

When parsing the "key", "command", and "cmd" trailer config, we just
make a copy of the value string.  If we see an implicit bool like:

  [trailer "foo"]
  key

we'll segfault trying to copy a NULL pointer. We can fix this with the
usual config_error_nonbool() check.

I split this out from the other vanilla cases, because at first glance
it looks like a better fix here would be to move the NULL check out of
the switch statement. But it would change the behavior of other keys
like trailer.*.ifExists, where an implicit bool is interpreted as
EXISTS_DEFAULT.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>