Additional clarifying assertion for streaming iteration

For OBJ_ITER_END, only call func() once

Motivated by #44

src/fellow_storage.c

@@ -782,6 +782,8 @@ 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)
+			AZ(l);
 	} while (l > 0);
 
 	fs->checkpoint = fs->written;