Avoid some valgrind warnings

Avoid some valgrind warnings

Using sscanf() to parse archive header values (struct ar_hdr) can lead
to valgrind warnings which are probably bogus but are annoying.
To be safer, create a local method to convert the ASCII integer
strings into integers.

* src/arscan.c (parse_int): Turn integer strings into integers.
(ar_scan): Initialize struct ar_hdr memory.
Call parse_int() rather than sscanf/atol.