Fix asn1_item_print_ctx

The call to asn1_do_adb can return NULL on error, so we should check the
return value before attempting to use it.
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/asn1/tasn_prn.c

@@ -285,6 +285,8 @@ static int asn1_item_print_ctx(BIO *out, ASN1_VALUE **fld, int indent,
         for (i = 0, tt = it->templates; i < it->tcount; i++, tt++) {
             const ASN1_TEMPLATE *seqtt;
             seqtt = asn1_do_adb(fld, tt, 1);
+            if(!seqtt)
+                return 0;
             tmpfld = asn1_get_field_ptr(fld, seqtt);
             if (!asn1_template_print_ctx(out, tmpfld,
                                          indent + 2, seqtt, pctx))