Wine: Crash when getting the text field of an empty TextBox/TextArea
When trying to set the text field of an empty TextArea the system crashes with a SIGSEGV

Log:
```
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui.nim(1012) addLine
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui.nim(2650) addLine
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui.nim(1011) addText
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui.nim(2648) addText
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui.nim(978) text
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui/private/windows/platform_impl.nim(1523) text
/home/chris/.nimble/pkgs/nigui-0.2.4/nigui/private/windows/platform_impl.nim(111) pGetWindowText
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
```

fixed this locally by adding the following check for `characters` in the pGetWindowText proc:

```
proc pGetWindowText(hWnd: pointer): string =
  let characters = GetWindowTextLengthW(hWnd)
  if characters == 0:
      return ""
  result = newString(characters * 2)
  var res = GetWindowTextW(hWnd, result, characters * 2 + 1)
  if res != characters: pRaiseLastOSError()
  result = result.pUtf16ToUtf8
```

Program runs fine on Manjaro, but crashes when running on wine. So the problem seems to be windows specific. After using the fix both versions run without problems.