Fix awful.screen.getbycoord when no screens exist

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

lib/awful/screen.lua

@@ -86,15 +86,18 @@ end
 -- @param x The x coordinate
 -- @param y The y coordinate
 function screen.getbycoord(x, y)
+    local dist = math.huge
     local s = capi.screen.primary
-    local dist = screen.object.get_square_distance(s, x, y)
+    if s then
+        dist = screen.object.get_square_distance(s, x, y)
+    end
     for i in capi.screen do
         local d = screen.object.get_square_distance(i, x, y)
         if d < dist then
             s, dist = capi.screen[i], d
         end
     end
-    return s.index
+    return s and s.index
 end
 
 --- Give the focus to a screen, and move pointer to last known position on this

spec/awful/screen_spec.lua

@@ -76,6 +76,16 @@ describe("awful.screen", function()
             assert.is.equal(2, ascreen.getbycoord(1, 1))
         end)
     end)
+
+    describe("no screens", function()
+        before_each(function()
+            fake_screens = {}
+        end)
+
+        it("getbycoord", function()
+            assert.is_nil(ascreen.getbycoord(0, 0))
+        end)
+    end)
 end)
 
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