[fixed] UV - vertex / tighten

[Hank]

Hi there,

Steps:

1. add any object
2. select face + uv mapping
3. do uv + unfold
4. for uv window select any vertex + tighten

[UV] vertex / tighten is causing crash_dump:

Dump written 2018-11-30_12-46
Version: 2.1.7
OS: Windows 8 (build 9200), 64-bit edition
GPU: Intel | Intel® HD Graphics 520
Window: {autouv,68}
Reason: function_clause

Short stack trace:
[{wings_drag,translate_fun,2,
[{file,"c:/Users/familjen/src/wings/src/wings_drag.erl"},
{line,172}]},
{wings_drag,fold_1,3,
[{file,"c:/Users/familjen/src/wings/src/wings_drag.erl"},
{line,205}]},
{wings_drag,fold,4,
[{file,"c:/Users/familjen/src/wings/src/wings_drag.erl"},
{line,118}]},
{wpc_autouv,handle_command,2,
[{file,"c:/Users/familjen/src/wings/plugins_src/autouv/wpc_autouv.erl"},
{line,785}]},
{wings_wm,handle_event,3,
[{file,"c:/Users/familjen/src/wings/src/wings_wm.erl"},{line,972}]},
{wings_wm,send_event,2,
[{file,"c:/Users/familjen/src/wings/src/wings_wm.erl"},{line,939}]},
{wings_wm,do_dispatch,2,
[{file,"c:/Users/familjen/src/wings/src/wings_wm.erl"},{line,840}]},
{wings_wm,dispatch_event,1,
[{file,"c:/Users/familjen/src/wings/src/wings_wm.erl"},{line,747}]},
{wings_wm,get_and_dispatch,0,
[{file,"c:/Users/familjen/src/wings/src/wings_wm.erl"},{line,651}]},
{wings,init_part2,2,
[{file,"f:/unixlike/src/wings/src/wings.erl"},{line,104}]},
{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]

BR, Hank