efi_loader: AllocateAdress error handling

If AllocatePages() is called with AllocateAddress, the UEFI spec requires
to return EFI_NOT_FOUND in case the memory page does not exist.

The UEFI SCT II 2017 spec additionally requires to return EFI_NOT_FOUND if
the page is already allocated.

Check that *Memory refers to an unallocated page.

UEFI SCT II (2017): AllocatePages(), 5.1.2.1.9 - 5.1.2.1.10

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>