pxelinux: cleaner test for OACK trailing null bytes

pxelinux: cleaner test for OACK trailing null bytes

Instead of looking for a string of null bytes at the end of the OACK
string, simply abort parsing if we run into a null byte where an
option is expected; either we are seeing junk at the end of the
packet, or we are hopelessly confused about how to make sense of the
rest of the packet -- in either case, ignoring is the "liberal"
option.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>