Fix memset issue in highlighting

[CakesStuff]

The previous memset used an incorrect variable causing various issues with single line comments.

[BugByteBender]

Oh, dear @CakesStuff did you say about this?

This should use row->rsize instead of row->size, since row->hl is allocated based on row->rsize (the rendered size), not the original size. The fix is simply using the correct length - no null termination needed since this isn't string data.

Someone should apply this patch, because this can occur during normal editing operations.