Fixed handling of ngx_write_fd() and ngx_read_fd() errors.

The ngx_write_fd() and ngx_read_fd() functions return -1 in case of error,
so the incorrect comparison with NGX_FILE_ERROR (which is 0 on windows
platforms) might result in inaccurate error message in the error log.

Also the ngx_errno global variable is being set only if the returned value
is -1.