Fix T67777: Linked rig crashes Blender 2.8 RC3 on File|New.

Sync back between proxy armature pose and linked one was not properly
clearing the posebone runtime data, ending up with shared memory between
both (direct cause of reported crash), and probably some other nasty
issues.

While this crash is not critical in itself, I would suggest to add this
fix to 2.80 branch, as it is super-safe and simple, and is probably
fixing some other un-reported issues?

source/blender/blenkernel/intern/armature.c

@@ -2403,6 +2403,9 @@ static void pose_proxy_synchronize(Object *ob, Object *from, int layer_protected
       pchanw.mpath = pchan->mpath;
       pchan->mpath = NULL;
 
+      /* Reset runtime data, we don't want to share that with the proxy. */
+      BKE_pose_channel_runtime_reset(&pchanw.runtime);
+
       /* this is freed so copy a copy, else undo crashes */
       if (pchanw.prop) {
         pchanw.prop = IDP_CopyProperty(pchanw.prop);