awful.tag.object.set_screen: Cope with screen being nil (#2202)

For "stuff around screen's 'removed' signal", it might make sense to
temporarily set a tags screen to nil. The idea is that it will only
later be assigned to a new screen, not immediately.

However, currently a tag with screen nil causes quite some problems in
the set_screen() function. This commit works around this with a generous
amount of "wrap this in if".

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/tag.lua

@@ -500,7 +500,9 @@ function tag.object.set_screen(t, s)
 
     -- Change the screen
     tag.setproperty(t, "screen", s)
+    if s then
         tag.setproperty(t, "index", #s:get_tags(true))
+    end
 
     -- Make sure the client's screen matches its tags
     for _,c in ipairs(t:clients()) do
@@ -508,6 +510,7 @@ function tag.object.set_screen(t, s)
         c:tags({t})
     end
 
+    if old_screen then
         -- Update all indexes
         for i,t2 in ipairs(old_screen.tags) do
             tag.setproperty(t2, "index", i)
@@ -518,6 +521,7 @@ function tag.object.set_screen(t, s)
             tag.history.restore(old_screen, 1)
         end
     end
+end
 
 --- Set a tag's screen
 -- @deprecated awful.tag.setscreen