[counters] Fix file handle leak in cpu_load ()

If a premature return is taken, the FILE pointer f is not fclose()-ed.

Report and fix provided by Markus Beth on the mailing list: http://lists.ximian.com/pipermail/mono-devel-list/2014-December/042459.html
Merge pull request #1434 from akoeplinger/fix-filehandle-leak

[counters] Fix file handle leak in cpu_load ()