[Bug 55717] mono's IsOutlineVisible_Line_WithGraphics test fails

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=55717

--- Comment #3 from Esme Povirk  ---
(In reply to Bartosz from comment #2)
> Created attachment 75307 [details]
> Potential fix for GdipIsVisiblePathPoint
>
> @Jeff Maybe you would like to proceed with the fix.
>
> It should be trivial to check if image is set (see attachment).

This change fixes IsOutlineVisible_Line_WithGraphics, but it also breaks
IsOutlineVisible_Line_WithGraphics_PageUnit and
IsOutlineVisible_Line_WithGraphics_Transform, which had been fixed by the same
patch that caused this regression.

We also need to NULL-check graphics before accessing graphics->image, otherwise
it crashes in Wine tests.