tree-wide: return NULL from freeing functions

I started working on this because I wanted to change how
DEFINE_TRIVIAL_CLEANUP_FUNC is defined. Even independently of that change, it's
nice to make make things more consistent and predictable.

src/basic/strbuf.h

@@ -32,8 +32,8 @@ struct strbuf_child_entry {
         struct strbuf_node *child;
 };
 
-struct strbuf *strbuf_new(void);
+struct strbuf* strbuf_new(void);
 ssize_t strbuf_add_string(struct strbuf *str, const char *s, size_t len);
 void strbuf_complete(struct strbuf *str);
-void strbuf_cleanup(struct strbuf *str);
+struct strbuf* strbuf_cleanup(struct strbuf *str);
 DEFINE_TRIVIAL_CLEANUP_FUNC(struct strbuf*, strbuf_cleanup);