Avoid overwriting slotid with stack garbage

[botovq]

When the calculation of slotid was converted to piv_slotid_from_string() a piece of the previous strotul() parsing was left behind in f73d2a76 overwriting the just computed slotid with stack garbage.

[botovq]

There are a few more warnings, but as they aren't about actual bugs, I refrained from touching those.

piv-certs.c:2556:23: note: uninitialized use occurs here
        freezero(sig, siglen);
                      ^~~~~~
piv-certs.c:2508:9: note: remove the 'if' if its condition is always false
        if (rc != 1) {
        ^~~~~~~~~~~~~~
piv-certs.c:2494:30: note: initialize the variable 'siglen' to silence this warning
        size_t tbslen, siglen;
                             ^
                              = 0

pivy-box.c:185:13: warning: variable 'parsed' is uninitialized when used here [-Wuninitialized]
                        slotid = parsed;
                                 ^~~~~~
pivy-box.c:132:21: note: initialize the variable 'parsed' to silence this warning
        unsigned int parsed;
                           ^
                            = 0

pivy-agent.c:2065:7: warning: unused variable 'i' [-Wunused-variable]
        uint i;
             ^
pivy-agent.c:2066:8: warning: unused variable 'f' [-Wunused-variable]
        FILE *f;
              ^

[arekinath]

Thanks! That's an embarrassing one. I'll go through those other warnings, too. Looks like I need to build with clang more often between releases.