some platforms

Since the behavior of write(fd, buf, 0) is undefined, it may fail with
EFAULT on some architectures when buf == NULL. The error was propagated
up to a caller, since my_write() code did not handle it properly.

Fixed by checking the 'number of bytes' argument in my_write() and
returning before calling the write() system call when there is nothing
to write.


mysys/my_write.c:
  Return from my_write() before calling the write() system call when the
  number of bytes to be written is 0, since the behavior of write() in
  this case is not portable.