Fix a compiler warning.

Fix a compiler warning.

input.c: In function ‘load_input_file’:
input.c:123:7: warning: ‘fd’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   len = read(fd, buf->str, BUFSIZE);
       ^
input.c:42:6: note: ‘fd’ was declared here
  int fd;
      ^