Fix out-of-bounds error when incorporating email from stdin

Fix out-of-bounds error when incorporating email from stdin

Before this patch, if +inbox is empty, following error happened:

	$ inc -file - < /dev/null

	Incorporating new mail into inbox...

	inc: no messages incorporated, continuing...
	inc: Bug: message out of bounds

This happened due improper call to `seq_setunseen', which implicitly
assumed that there is at least one message (it does not handle case
where both mp->hghmsg and mh->lowmsg are 0).

Bug does not happens with command `inc -file empty', since file
is checked for being empty before any read attempts.

Signed-off-by: Dmitry Bogatov <KAction@gnu.org>