efi_loader: efi_current_var after SetVirtualAddressMap

Variable efi_current_var is a pointer to a physical memory address that
becomes invalid after SetVirtualAddressMap(). Instead of converting it via
ConvertPointer() simply set it to NULL.

Fixes: b02a7071 ("efi_loader: enable UEFI variables at runtime")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>