sparc: Fix assertion caused by empty memory slot with 0 byte

sparc: Fix assertion caused by empty memory slot with 0 byte

If the memory size given on the command line is equal to the
maximum size of memory defined by the hardware, there is no
"empty slot" after physical memory.

The following command

		qemu-system-sparc -m 256

raised an assertion:
exec.c:2614: cpu_register_physical_memory_offset: Assertion `size' failed

This can be fixed either at the caller side (don't call empty_slot_init)
or in empty_slot_init (do nothing) when size == 0. The second solution
was choosen here because it is more robust.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>