fellow_cache: Add assertion that seglists have at least one member

src/fellow_cache.c

@@ -2861,6 +2861,7 @@ fellow_disk_seglist_alloc(struct fellow_busy *fbo,
 	AZ(ofdsl->next.off);
 	AZ(ofdsl->next.size);
 
+	AN(ldsegs);
 	sz = SEGLIST_SIZE(fdsl, ldsegs);
 
 	fdr = fellow_busy_region_alloc(fbo, sz, 0);