Fix get_int_via_sysctlbyname(..) on Jenkins

Initialize oldlen to the size of the value, instead of leaving the value
unitialized. Leaving it unitialized seems to work by accident on amd64 when
running 64-bit programs, but not on i386.

This matches patterns in use in other programs.

PR: 237458
Approved by: emaste (mentor; implicit)
MFC after: 1 week
Tested on: ^/head (amd64), ^/stable/11 (i386)