Convert size_t's used in conjuction with sk_X509_num back to int.

Convert size_t's used in conjuction with sk_X509_num back to int.

The lets the regress in x509/policy pass instead of infinite looping.

The changes are necessry  because our sk_num() returns an int with
0 for empty and -1 for NULL, wheras BoringSSL's returns a size_t with
0 for both an empty stack and a NULL stack.

pair work with tb@
ok tb@ jsing@