common/sphinx: eliminate temporary buffer for prefill.

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

common/sphinx.c

@@ -452,15 +452,13 @@ static void sphinx_prefill(u8 *routinginfo, const struct sphinx_path *sp,
 	size_t last_hop_size = sphinx_hop_size(&sp->hops[num_hops - 1]);
 	int prefill_offset =
 	    ROUTING_INFO_SIZE - fillerSize - last_hop_size - prefill_size;
-	u8 prefill[prefill_size];
 	struct secret shared_secret;
 
 	/* Generate the prefill stream, which cancels out the layers of
 	 * encryption that will be applied while wrapping the onion. This
 	 * leaves the middle, unused, section with all 0x00 bytes after
 	 * encrypting. */
-	generate_prefill(prefill, prefill_size, sp, params);
-	memcpy(routinginfo + prefill_offset, prefill, prefill_size);
+	generate_prefill(routinginfo + prefill_offset, prefill_size, sp, params);
 
 	/* Now fill in the obfuscation stream, which can be regenerated by the
 	 * node processing this onion. */