[Windows] Selecting *.* causes save dialog to append entire list of last selected file formats

[btzy]

Steps to reproduce:

1. Run test_savedialog.c.
2. Change the filter dropdown to *.*.
3. Type a file name without extension (e.g. "test"), and click "Save" / press Enter.
4. Observe that the returned file name is "C:\path\to\file\test.c,cpp,cc".

Expected behaviour:

It should return "C:\path\to\file\test.c" (i.e. just the first extension) instead. Note that normal Windows behaviour seems to append the ".c" extension even if the filter dropdown is *.*, so we should stick to that.

Reference: mlabbe/nativefiledialog#101