fix is_mounted (memory leaks and NULL access)

[jmtd]

Tomm: please disregard my previous pull request, which was bad. (Sorry, rusty at C). This one works :-)

I experimented with stack-allocated fixed buffers for use with realpath but unfortunately PATH_MAX is not portable.

Ensure that the return of realpath is checked for NULL before access. Ensure that the return value is free'd before it falls out of scope. Related tidying up.