stream: fix sequence number on smsg

When multiple segments were put into a smsg, the seq would be updated
each time a segment was added. Because of this, the seq wasn't pointing
to the start of the data.

This caused some false negatives when the fast_pattern was in the raw
stream, but another part of the inspection was in the state. Because of
the wrong seq, the inspection of the smsg could be delayed. This in turn,
could make the inspection engine consider a TX inspected, even if it wasn't
fully yet.