Prefix crlNumber output with 0x.

When the crlNumber field contains only 0-9 digits, the output is
ambiguous as to what base it's in, which can be confusing. Adding this
prefix makes it explicit that it's in hex.

CLA: trivial

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12434)

apps/crl.c

@@ -288,6 +288,7 @@ int crl_main(int argc, char **argv)
                 crlnum = X509_CRL_get_ext_d2i(x, NID_crl_number, NULL, NULL);
                 BIO_printf(bio_out, "crlNumber=");
                 if (crlnum) {
+                    BIO_puts(bio_out, "0x");
                     i2a_ASN1_INTEGER(bio_out, crlnum);
                     ASN1_INTEGER_free(crlnum);
                 } else