Fix memory leak in vmstat

[trix_juniper.net]

Summary

vmstat.c dointr
Free memory assigned to intrnames before returning.
Free memory assigned to intrcnts before breaking out of while loop. Move assignment of old_intrcnts to after the break check because it isn't needed before the check.

Test Plan

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