Fix the efi serial console in the Arm models.

On some UEFI implementations the ConsOut EFI variable is not a device
path end type so we never move to the next node. Fix this by always
incrementing the device path node pointer, with a sanity check that
the node length is large enough so no two nodes overlap.

While here return failure on malloc failure rather than a NULL pointer
dereference.

Reviewed by: tsoome, imp (previous version)
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D25202