gdi32/dibdrv: don't add_join if points are coincident in wide_line_segments (try 2)

[Daniel Lehman]

try 2 - integrate into existing tests; updated commit message

I think the only points in the middle of the points array can cause the crash I was seeing.  wide_pen_lines trims coincident points/empty segments from the beginning and end of the array, so the wine_line_segment calls before and after the for loop I modified will always return valid faces