Remove asserts from addr_validate_path_internal()

Remove asserts from addr_validate_path_internal()

This is reachable from x509_verify(), but all asserts are previously
checked in the caller. Turn them into error checks and make sure
the error is set on the X509_STORE_CTX if present. Change some
stack == NULL || sk_num(stack) == 0 checks into sk_num(stack) <= 0
which is equivalent but simpler.

ok jsing