Fix segfault on exit (introduced by the new backing store code)

A small typo in a function argument and C compiler happily accepting
void pointers instead of something else is a dangerous combo. Need to
be more careful.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

src/fbdev.c

@@ -1110,7 +1110,7 @@ FBDevCloseScreen(CLOSE_SCREEN_ARGS_DECL)
 	}
 
 	if (fPtr->backing_store_tuner_private) {
-	    BackingStoreTuner_Close(fPtr->backing_store_tuner_private);
+	    BackingStoreTuner_Close(pScreen);
 	    free(fPtr->backing_store_tuner_private);
 	    fPtr->backing_store_tuner_private = NULL;
 	}