cmake: add valgrind target

Don't include this in the check target, since valgrind is quite a build
requirement *and* only works really well on Linux.
tests: allocate enough space for track_offsets

Not only the track offsets, but also the overal disc sector count is
saved in this array.
valgrind: also check for leaks

The default does display a summary, but we suppress this summary and it
is normally not indicated with an error status.
So we force returning an error code for leaks.
fix a leak in the tests

This is the memory problem I actually found first,
but valgrind pushed me to fix another one first ;-)
cmake: rename valgrind target to memcheck