Refactor confusing code in _mdfd_openseg().

As reported independently by a couple of people, _mdfd_openseg() is coded in a
way that seems to imply that the segments could be opened in an order that
isn't strictly sequential.  Even if that were true, it's also using the wrong
comparison.  It's not an active bug, since the condition is always true anyway,
but it's confusing, so replace it with an assertion.

Author: Thomas Munro
Reviewed-by: Andres Freund, Kyotaro Horiguchi, Noah Misch
Discussion: https://postgr.es/m/CA%2BhUKG%2BNBw%2BuSzxF1os-SO6gUuw%3DcqO5DAybk6KnHKzgGvxhxA%40mail.gmail.com
Discussion: https://postgr.es/m/20191222091930.GA1280238%40rfd.leadboat.com

src/backend/storage/smgr/md.c

@@ -1100,8 +1100,13 @@ _mdfd_openseg(SMgrRelation reln, ForkNumber forknum, BlockNumber segno,
 	if (fd < 0)
 		return NULL;
 
-	if (segno <= reln->md_num_open_segs[forknum])
-		_fdvec_resize(reln, forknum, segno + 1);
+	/*
+	 * Segments are always opened in order from lowest to highest, so we must
+	 * be adding a new one at the end.
+	 */
+	Assert(segno == reln->md_num_open_segs[forknum]);
+
+	_fdvec_resize(reln, forknum, segno + 1);
 
 	/* fill the entry */
 	v = &reln->md_seg_fds[forknum][segno];