Fix possible use of invalid pointer.

It was possible to use an invalid pointer to get the target ID value. To fix
this, initialize a local Target ID variable to an invalid value and change that
variable to a valid value only if the pointer to the Target ID is not NULL.

Reviewed by: ken, scottl, ambrisko, asomers
Approved by: ken, scottl, ambrisko
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6100