In prom.c we run finish_node() on allnodes twice. The first time we just
calculate how much memory we'll need, the second time we do the actual work.

If the calculation stage determines that we need 0 bytes, then we should skip
the lmb allocation. Although an alloc of zero will work, it has been seen to
lead to a BUG_ON() in reserve_bootmem() on at least one machine.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>