Silence dead assigmnents/initialization/increments

Some of these assignments were introduced in commit
c9d5cd059c45fd3aa6d16b3ba185d7cb3a08de9e ("hivex: Fix allocations that
may move C heap buffer."), but turn out to be unnecessary (unused).
Replace the assignments by a comment in case someone extends the code
later.

Caught by Clang static analyzer.
Avoid calling calloc(0, x)

The return value for zero-size allocations is implementation-defined
(can be NULL or a unique pointer for free()). Just return early here as
there is nothing to print.

Caught by Clang static analyzer.