awful.wibar: move wibar:remove() call outside of loop over all wiboxes (#1501)

Removing a wibar mid-loop will corrupt the indexing of the for loop and can
cause other wibars from being removed.

lib/awful/wibar.lua

@@ -359,11 +359,15 @@ function awfulwibar.new(arg)
 end
 
 capi.screen.connect_signal("removed", function(s)
+    local wibars = {}
     for _, wibar in ipairs(wiboxes) do
         if wibar._screen == s then
-            wibar:remove()
+            table.insert(wibars, wibar)
         end
     end
+    for _, wibar in ipairs(wibars) do
+        wibar:remove()
+    end
 end)
 
 function awfulwibar.mt:__call(...)