Ensure that empty e820 regions are skipped even at the end of the

Ensure that empty e820 regions are skipped even at the end of the
memory map.  (We achieve this by setting CF on the last entry if it is
zero-length; this avoids the need to look ahead to see at each entry
if the *next* entry would be both the last entry and zero-length).

This fixes the "0kB base memory" error message upon starting Windows
2003 on a SunFire X2100.