util/streaming-buffer: fix regions intersection

util/streaming-buffer: fix regions intersection

This was not a problem for current callers in Suricata,
as RegionsIntersect is only called through StreamingBufferInsertAt
which is only used by TCP...

And TCP uses default region gap = 256kb, and only calls
StreamingBufferInsertAt with a u16, so TCP never inserts a new
data that will strictly contain an existing region augmented
with region gap, which was the only case where RegionsIntersect
returned the wrong result, which could later lead to a
buffer overflow.

Ticket: 7393