Fixed slight memory leak in sf::Font

[dermoumi]

Nothing much, but apparently it's all i'm good at 😞 ( see #509 )

There is also an inconsistency in the coding style; Either adding objects to the class as soon as they're created so that cleanup() frees them eventually, as is the case with the stroker object, which is cleaner IMO. Or adding them at the very end and having to clean every single one whenever there's an error loading, as it's the case with original code, this frees memory instantly but irrelevant if there's no errors (most of the time).

But I did not touch that.

[LaurentGomila]

You're right, I don't like these functions, we should invest some time trying to make them cleaner and more robust.

In the meantime, your fix is ok, and I think you can also add a call to FT_Done_Face(face) when FT_Stroker_New fails.

[dermoumi]

There! I hope this will do.

[eXpl0it3r]

Squashed and merged the commits in c43b599