pxe: Tidy up code style a little in pxe_utils

There are a few more blank lines than makes sense for readability. Also
free() handles a NULL pointer so drop the pointless checks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>