sign changes due to negative values being implicitly cast to `ssize_t`

[agustin-vaca]

In parse-args.c, lines 1402, 1404, and 1406, there are negative values being implicitly cast to ssize_t, which causes their sign to change.

@plavin
https://github.com/hpcgarage/spatter/blame/main/src/parse-args.c#L1402

Possible solutions:

1. Changing to int (not the most portable)
2. Changing to ptrdiff_t

[agustin-vaca]

Fixed with PR #145