RFC: comdlg32: Fix null lpstrFile uses in FILEDLG95_OnOpen

[Bruno Jesus]

Hi, I have written a patch to fix bug 17229 and would like to request
help to complete the test part.

The test creates a common dialog and select a filename (file does not
need to exist), it was copied from other tests in the same file
(comdlg32/tests/filedlg.c). It's not possible to have a default file
name because lpstrFile is used to do this and it's NULL. To fill a
filename in the common dialog control that appears I have used
EnumChildWindows to find the textbox and set the filename in it. The
main problem is how to detect that a HWND is a textbox? Currently I'm
sending SetWindowText to every control and that does work but I think
the patch may be rejected by that.

The patch can be downloaded at:
https://testbot.winehq.org/JobDetails.pl?Key=19074

The problem is in the line 56.

Thanks in advance,
Bruno