Remove various compilation warnings

With the removal of these warnings we slightly improve the robustness of
the highscore parsing, handling some more cases of bad parsing/writing
and failure to open files.

src/main.c

@@ -14,7 +14,7 @@ void draw_then_sleep(struct gfx_state *s, struct gamestate *g)
 int main(int argc, char **argv)
 {
     struct gamestate *g = gamestate_init(argc, argv);
-    struct gfx_state *s;
+    struct gfx_state *s = NULL;
 
     if (g->opts->interactive)
         s = gfx_init(g);