[fixed] UV - vertex / tighten

[Hank]

Hi Micheus,

I've just started to work with 2.2.1 and the problem still exist:
(this time I tested with face -> tighten)

Dump written 2018-12-7_17-52
Version: 2.2.1
OS: Linux 4.15.0-20-generic x86_64
GPU: NVIDIA Corporation | GeForce GTX 650 Ti/PCIe/SSE2
Window: {autouv,1}
Reason: function_clause

Short stack trace:
[{wings_drag,translate_fun,2,[{file,"wings_drag.erl"},{line,178}]},
{wings_drag,fold_1,3,[{file,"wings_drag.erl"},{line,211}]},
{wings_drag,fold,4,[{file,"wings_drag.erl"},{line,124}]},
{wpc_autouv,handle_command,2,[{file,"wpc_autouv.erl"},{line,785}]},
{wings_wm,handle_event,3,[{file,"wings_wm.erl"},{line,981}]},
{wings_wm,send_event,2,[{file,"wings_wm.erl"},{line,948}]},
{wings_wm,do_dispatch,2,[{file,"wings_wm.erl"},{line,849}]},
{wings_wm,dispatch_event,1,[{file,"wings_wm.erl"},{line,758}]},
{wings_wm,get_and_dispatch,0,[{file,"wings_wm.erl"},{line,657}]},
{wings,init_part2,2,[{file,"wings.erl"},{line,106}]},
{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]

Long stack trace:
[{wings_drag,translate_fun,

BR, Hank