Handle validation of inputs in efi_loadopt_create() better.

Handle validation of inputs in efi_loadopt_create() better.

efibootmgr tries to tell how much space it needs to allocate by passing
in optional_data=NULL, optional_data_size=$SOMESIZE, and we're
validating that both things are set way too early.

Move that until after the "if (size==0) return sz;" check.

Based on the bug report in issue#22 and pull#27 .

Signed-off-by: Peter Jones <pjones@redhat.com>