fellow_stream_f(): Improve comment and assertion

to make clear that we understand exactly what is happening.

src/fellow_storage.c

@@ -754,9 +754,6 @@ fellow_stream_f(void *priv, unsigned flush, const void *ptr, ssize_t len)
 	assert(fs->available >= fs->written);
 	l = vmin_t(ssize_t, len, fs->available - fs->written);
 
-	if (flush & OBJ_ITER_END)
-		assert(l <= len);
-
 	do {
 		r = fs->func(fs->priv, flush, p, l);
 		if (r)
@@ -772,7 +769,15 @@ fellow_stream_f(void *priv, unsigned flush, const void *ptr, ssize_t len)
 
 		assert(fs->available >= fs->written);
 		l = vmin_t(ssize_t, len, fs->available - fs->written);
-	} while (l > 0);	// XXX len > 0 ?
+	} while (l > 0);
+
+	/*
+	 * can only have unsent data if the at the last segment and it was
+	 * FCS_BUSY, so len is the allocation size, not the used size
+	 */
+	if (len > 0)
+		assert(flush & OBJ_ITER_END);
+
 	fs->checkpoint = fs->written;
 	return (r);
 }