ctf: assert that msg iter and medium seek offset is valid

ctf: assert that msg iter and medium seek offset is valid

I don't think there is ever a valid situation for `offset` to have an
invalid value in ctf_msg_iter_seek or medop_seek.  If the user of the
iterator (e.g.  fs.c) thinks it's cool enough to seek, it should know
about the underlying medium it used to create the msg iter and make sure
not to seek at an invalid position.

This allows removing the INVAL values in enum ctf_msg_iter_medium_status
and enum ctf_msg_iter_status.

Change-Id: I19d48fe751aebf21e60c5cbc16127919a5fa72f3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>