efi_loader: return NULL from device path functions

For the construction of device paths we need to call the
AllocatePool service. We should not ignore if it fails due to an
out of memory situation.

This patch changes the device path functions to return NULL if
the memory allocation fails.

Additional patches will be needed to fix the callers.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>