To prevent problems later in boot, make sure we don't create zero-size lmb
regions.

I've checked all the callers, and at the moment no one should ever hit this.
All callers use a constant size, or they check the computed size before they
call us.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>