migration: Fix compiler warning ('caps' may be used uninitialized)

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Weil <sw@weilnetz.de>

migration.c

@@ -150,6 +150,7 @@ MigrationCapabilityStatusList *qmp_query_migrate_capabilities(Error **errp)
     MigrationState *s = migrate_get_current();
     int i;
 
+    caps = NULL; /* silence compiler warning */
     for (i = 0; i < MIGRATION_CAPABILITY_MAX; i++) {
         if (head == NULL) {
             head = g_malloc0(sizeof(*caps));