efi_loader: initialize device path on alloc

Since the backing memory for a new device path can contain stale
data we have to make sure that we zero the buffer.  Otherwise some
code paths that don't set all fields in a structure backed by this
device path might contain unwanted stale data.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>