criu: pstree_switch_state -- Fix nil deref on error path
criu: pstree_switch_state -- Fix nil deref on error path

We may enter with @root_item = NULL in case of error
handling, so simply bail out.

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,eabatalov/criu,eabatalov/criu,efiop/criu,AuthenticEshkinKot/criu,AuthenticEshkinKot/criu,efiop/criu,AuthenticEshkinKot/criu,eabatalov/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote,eabatalov/criu,efiop/criu,AuthenticEshkinKot/criu,efiop/criu,eabatalov/criu,efiop/criu,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,KKoukiou/criu-remote