memento_softwareupdate: fix split_write error "bad file descriptor"

split_write.cc

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <sys/stat.h>
 #include <sys/types.h>
 
 #include <endian.h>
 #include <fcntl.h>
 #include <stdio.h>
@@ skipping 59 matching lines @@
 }
 
 // Returns valid fd or exits program.
 int open_file(const char* path) {
   int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
   if (fd < 0) {
     perror("open");
     fprintf(stderr, "failed to open %s\n", path);
     exit(1);
   }
+  return fd;
 }
 
 typedef long long int64;
 // Compile assert sizeof(int64) == 8:
 char int64_8_bytes_long[(sizeof(int64) == 8) - 1];
 
 }  // namespace {}
 
 int main(int argc, char** argv) {
   if (argc != 3) {
@@ skipping 38 matching lines @@
   }
   // Do the rest on the second file
   for (;;) {
     size_t chunk_size = read_all(fd_in, buf, kBufSize);
     if (chunk_size == 0)
       break;
     write_all(fd2, buf, chunk_size);
   }
   return 0;
 }