plugins: Initialize plugins control by default

We run "pre-dump" scripts earlier than calling
anything else, so in case if it get failed the
plugins cleanup routine hit list structure set
into zeros which lead into SIGSEGV when we're
trying to walk over.

Thus simply init it as empty list.
Reported-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/plugin.c

@@ -13,7 +13,10 @@
 #include "list.h"
 #include "log.h"
 
-cr_plugin_ctl_t cr_plugin_ctl;
+cr_plugin_ctl_t cr_plugin_ctl = {
+	.head.next = &cr_plugin_ctl.head,
+	.head.prev = &cr_plugin_ctl.head,
+};
 
 /*
  * If we met old version of a plugin, selfgenerate a plugin descriptor for it.