processReauthTags sets UserID when converting a tagged node to
user-owned, but does not set the User pointer. When the node was
registered with a tags-only PreAuthKey (User: nil), the in-memory
NodeStore cache holds a node with User=nil. The mapper's
generateUserProfiles then calls node.Owner().Model().ID, which
dereferences the nil pointer and panics.
Set node.User alongside node.UserID in processReauthTags. Also add
defensive nil checks in generateUserProfiles to gracefully handle
nodes with invalid owners rather than panicking.
Fixes#3038
80a34ec3c1