mouse: Avoid an error when the client is killed while moving

to reproduce:

1) spawn an xterm
2) enter 'sleep 10 && killall xterm'
3) start moving the terminal

There will be an error

(found by a yet to be commited integration test)

lib/awful/layout/suit/floating.lua

@@ -41,6 +41,8 @@ function floating.mouse_resize_handler(c, corner, x, y)
     end
 
     capi.mousegrabber.run(function (_mouse)
+                              if not c.valid then return false end
+
                               _mouse.x = _mouse.x + coordinates_delta.x
                               _mouse.y = _mouse.y + coordinates_delta.y
                               for _, v in ipairs(_mouse.buttons) do

lib/awful/layout/suit/magnifier.lua

@@ -30,6 +30,8 @@ function magnifier.mouse_resize_handler(c, corner, x, y)
 
     local prev_coords = {}
     capi.mousegrabber.run(function (_mouse)
+                              if not c.valid then return false end
+
                               for _, v in ipairs(_mouse.buttons) do
                                   if v then
                                       prev_coords = { x =_mouse.x, y = _mouse.y }

lib/awful/layout/suit/tile.lua

@@ -85,6 +85,8 @@ local function mouse_resize_handler(c, _, _, _, orientation)
 
     local prev_coords = {}
     capi.mousegrabber.run(function (_mouse)
+                              if not c.valid then return false end
+
                               _mouse.x = _mouse.x + coordinates_delta.x
                               _mouse.y = _mouse.y + coordinates_delta.y
                               for _, v in ipairs(_mouse.buttons) do

lib/awful/mouse/init.lua

@@ -174,6 +174,8 @@ function mouse.client.move(c, snap, finished_cb)
     local fixed_y = c.maximized_vertical
 
     capi.mousegrabber.run(function (_mouse)
+                              if not c.valid then return false end
+
                               for _, v in ipairs(_mouse.buttons) do
                                   if v then
                                       local lay = layout.get(c.screen)
@@ -218,6 +220,8 @@ mouse.client.dragtotag = { }
 -- @param c The client to move
 function mouse.client.dragtotag.border(c)
     capi.mousegrabber.run(function (_mouse)
+                                if not c.valid then return false end
+
                                 local button_down = false
                                 for _, v in ipairs(_mouse.buttons) do
                                     if v then button_down = true end