migration: use parameters.mode in cpr_state_save

qmp_migrate guarantees that cpr_channel is not null for
MIG_MODE_CPR_TRANSFER when cpr_state_save is called:

    qmp_migrate()
        if (s->parameters.mode == MIG_MODE_CPR_TRANSFER && !cpr_channel) {
            return;
        }
        cpr_state_save(cpr_channel)

but cpr_state_save checks for mode differently before using channel,
and Coverity cannot infer that they are equivalent in outgoing QEMU,
and warns that channel may be NULL:

    cpr_state_save(channel)
        MigMode mode = migrate_mode();
        if (mode == MIG_MODE_CPR_TRANSFER) {
            f = cpr_transfer_output(channel, errp);

To make Coverity happy, assert that channel != NULL in cpr_state_save.

Resolves: Coverity CID 1590980
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Message-ID: <1738788841-211843-1-git-send-email-steven.sistare@oracle.com>
[assert instead of using parameters.mode in cpr_state_save]
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/cpr.c

@@ -137,6 +137,7 @@ int cpr_state_save(MigrationChannel *channel, Error **errp)
     trace_cpr_state_save(MigMode_str(mode));
 
     if (mode == MIG_MODE_CPR_TRANSFER) {
+        g_assert(channel);
         f = cpr_transfer_output(channel, errp);
     } else {
         return 0;