ocsp: corrected the comparison of the serial size in OCSP response

Previously the OCSP certificate check wouldn't verify the serial length
and could succeed in cases it shouldn't.

Reported by Stefan Buehler.

lib/x509/ocsp.c

@@ -1318,6 +1318,7 @@ gnutls_ocsp_resp_check_crt(gnutls_ocsp_resp_t resp,
 		gnutls_assert();
 		goto cleanup;
 	}
+	cserial.size = t;
 
 	if (rserial.size != cserial.size
 	    || memcmp(cserial.data, rserial.data, rserial.size) != 0) {