Remove asserts from asid_validate_path_internal()

Remove asserts from asid_validate_path_internal()

The first asserts ensure that things checked in the callers hold true.
Turn them into error checks and set the error on the X509_STORE_CTX
if it's present. Checking sk_value(..., i) with i < sk_num(...) isn't
useful, particularly if that check is done via an assert. Turn one
remaining assert into a NULL check. Finally, simplify the sk_num()
checks in the callers.

ok jsing