common/sphinx: use bigsize_get not varint_get.

These are not the same if it's more than one byte.  Testing would have
caught this, I assume.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/sphinx.c

@@ -486,7 +486,7 @@ static void sphinx_parse_payload(struct route_step *step, const u8 *src)
 		hop_size = FRAME_SIZE;
 		step->type = SPHINX_V0_PAYLOAD;
 	} else {
-		vsize = varint_get(src, 3, &raw_size);
+		vsize = bigsize_get(src, 3, &raw_size);
 		hop_size = raw_size + vsize + HMAC_SIZE;
 		step->type = SPHINX_TLV_PAYLOAD;
 	}