Fix a memory leak in libutil

[trix_juniper.net]

Summary

kinfo_getvmobject.c kinfo_getvmobject
When sysctlbyname fails, free buf before returning.

Test Plan

Use clang's static analyzer, scan-build, to find the problem and show resolution.

[jhb]

Hmm, I was going to suggest looking in other places in libutil that I copied this from, but it seems other kinfo_getfoo() functions don't use a loop to retry but just add a fudge factor to the size and hope it is enough.