Fix bad assertion in fellow_stream_f()

we can have FCS_BUSY segments before OBJ_ITER_END while streaming.

src/fellow_storage.c

@@ -771,13 +771,6 @@ fellow_stream_f(void *priv, unsigned flush, const void *ptr, ssize_t len)
 		l = vmin_t(ssize_t, len, fs->available - fs->written);
 	} 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);
 }