x86-64: fix combining of regions in init_memory_mapping()

When nr_range gets decremented, the same slot must be considered for
coalescing with its new successor again.

The issue is apparently pretty benign to native code, but surfaces as a
boot time crash in our forward ported Xen tree (where the page table
setup overall works differently than in native).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>