Fix compiler warning created in commit dd23b3b.

process_stats.c:189:6: warning: comparison of integers of different signs: 'unsigned long' and 'ssize_t' (aka 'long') [-Wsign-compare]
        if( PIPD_IS_FULL( pipd, sys_page_size ) ) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
process_stats.c:182:95: note: expanded from macro 'PIPD_IS_FULL'
                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^