util.crypto: Use stack space buffers

Removes assumption that LUAL_BUFFERSIZE is known at pre-processing time,
which it is not in Lua 5.3 and 5.4, where it is a computed macro based
on sizeof.

Allocation of stack space is safer and faster, no need to worry about
luaL_prepbuffer failing to allocate memory and skipping free()

util-src/crypto.c

@@ -434,7 +434,8 @@ static int Lparse_ecdsa_signature(lua_State *L) {
 	size_t sig_der_len;
 	const unsigned char *sig_der = (unsigned char*)luaL_checklstring(L, 1, &sig_der_len);
 	const BIGNUM *r, *s;
-	luaL_Buffer rb, sb;
+	unsigned char rb[32];
+	unsigned char sb[32];
 	int rlen, slen;
 
 	sig = d2i_ECDSA_SIG(NULL, &sig_der, sig_der_len);
@@ -449,23 +450,19 @@ static int Lparse_ecdsa_signature(lua_State *L) {
 	rlen = BN_num_bytes(r);
 	slen = BN_num_bytes(s);
 
-	// COMPAT w/ Lua 5.1
-	#if LUAL_BUFFERSIZE < 32
-	#error Configured LUAL_BUFFERSIZE is too small for this operation
-	#endif
-
-	luaL_buffinit(L, &rb);
-	BN_bn2bin(r, (unsigned char*)luaL_prepbuffer(&rb));
-	luaL_addsize(&rb, rlen);
-	luaL_pushresult(&rb);
+	if (rlen > 32 || slen > 32) {
+		ECDSA_SIG_free(sig);
+		luaL_error(L, "unexpectedly large signature integers");
+	}
 
-	luaL_buffinit(L, &sb);
-	BN_bn2bin(s, (unsigned char*)luaL_prepbuffer(&sb));
-	luaL_addsize(&sb, slen);
-	luaL_pushresult(&sb);
+	BN_bn2bin(r, rb);
+	BN_bn2bin(s, sb);
 
 	ECDSA_SIG_free(sig);
 
+	lua_pushlstring(L, (const char*)rb, rlen);
+	lua_pushlstring(L, (const char*)sb, slen);
+
 	return 2;
 }