qemu: Fix crash in tunnelled migration

Any attempt to start a tunnelled migration with libvirtd that supports
RDMA migration (specifically commit v1.2.8-226-ged22a474) crashes
libvirtd on the destination host.

The crash is inevitable because qemuMigrationPrepareAny is always called
with NULL protocol in case of tunnelled migration.

https://bugzilla.redhat.com/show_bug.cgi?id=1147331Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

src/qemu/qemu_migration.c

@@ -2707,7 +2707,7 @@ qemuMigrationPrepareAny(virQEMUDriverPtr driver,
                                        QEMU_MIGRATION_COOKIE_NBD)))
         goto cleanup;
 
-    if (STREQ(protocol, "rdma") && !vm->def->mem.hard_limit) {
+    if (STREQ_NULLABLE(protocol, "rdma") && !vm->def->mem.hard_limit) {
         virReportError(VIR_ERR_OPERATION_INVALID, "%s",
                        _("cannot start RDMA migration with no memory hard "
                          "limit set"));