odp-util: Fix buffer overread in parsing string form of ODP flows.

scan_u128() should return 0 on an error but it actually returned an errno
value in some cases, so a command like this:
    ovs-appctl dpctl/add-flow 'ct_label(1/55555555555555555555555555)' ''
could cause a buffer overread.

This bug is not as severe as it may sound because the string form of ODP
flows is not used over OpenFlow or OVSDB, only through the appctl interface
that is normally used just by local system administrators and not exposed
over a network.

Reported-by: Bhargava Shastry <bshastry@sec.t-labs.tu-berlin.de>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>

lib/odp-util.c

@@ -3022,7 +3022,7 @@ scan_u128(const char *s_, ovs_u128 *value, ovs_u128 *mask)
                 error = parse_int_string(s, (uint8_t *)&be_mask,
                                          sizeof be_mask, &s);
                 if (error) {
-                    return error;
+                    return 0;
                 }
                 *mask = ntoh128(be_mask);
             } else {