Use portable method of zeroing vector of ints.

The memset() technique is only applicable to platforms for which
numerical zero is represented by all bits zero.
Eliminate release-mode build warning.

The 'cbottom' variable is only used within an assert().  Don't define
the variable in release (i.e., "NDEBUG") mode.
Eliminate redundant explicit type conversion.

The pointers in question are already type 'int *'.  There is no need to
explicitly convert them to that type too.
Replace an assignment with intended equality test

This corrects a latent error that has been present since the inception
of this module.
Merge pull request #138 from bska/release-cleanup

Correct a number of latent issues in `tarjan.c`