Polish fellow_busy_body_seg_next()

The previous code

-       if (fdsl->nsegs == fcsl->lsegs ||
-           fcsl->segs[fdsl->nsegs].state == FCS_INIT) {

was confusing, it relied on the fact that, when an fcsl has more
segments than an fdsl, the first "surplus" segment has state FCS_INIT,
when it makes much more sense to just check fdsl->nsegs against
fdsl->lsegs.

src/fellow_cache.c

@@ -3986,15 +3986,17 @@ fellow_busy_body_seg_next(struct fellow_busy *fbo)
 	CHECK_OBJ_NOTNULL(fcsl, FELLOW_CACHE_SEGLIST_MAGIC);
 	fdsl = fcsl->fdsl;
 	CHECK_OBJ_NOTNULL(fdsl, FELLOW_DISK_SEGLIST_MAGIC);
-	assert(fdsl->nsegs <= fcsl->lsegs);
-	if (fdsl->nsegs == fcsl->lsegs ||
-	    fcsl->segs[fdsl->nsegs].state == FCS_INIT) {
+
+	assert(fdsl->nsegs <= fdsl->lsegs);
+	if (fdsl->nsegs == fdsl->lsegs) {
 		fcr = fellow_busy_body_seglist_alloc(fbo, fcsl);
 		if (fcr.status != fcr_ok)
 			return (fcr);
 		CAST_OBJ_NOTNULL(fcsl, fcr.r.ptr, FELLOW_CACHE_SEGLIST_MAGIC);
 		fbo->body_seglist = fcsl;
 		fdsl = fcsl->fdsl;
+		AZ(fdsl->nsegs);
+		AN(fdsl->lsegs);
 	}
 
 	CHECK_OBJ_NOTNULL(fdsl, FELLOW_DISK_SEGLIST_MAGIC);