loader/i386/xnu: Fix uninitialized scalar variable

loader/i386/xnu: Fix uninitialized scalar variable

In the function grub_xnu_boot(), struct grub_relocator32_state state is called
but isn't being initialized. This results in the members grub_uint32_t ebx,
grub_uint32_t ecx, grub_uint32_t edx, grub_uint32_t edi, and grub_uint32_t esi
being filled with junk data from the stack since none of them are being set to
any values. We can prevent this by setting state to {0}.

Fixes: CID 375035
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>