CLEANUP: use "offsetof" where appropriate

let's use the C library macro "offsetof"

Author: chipitsine

src/cache.c

@@ -425,12 +425,12 @@ static void delete_entry(struct cache_entry *del_entry)
 
 static inline struct shared_context *shctx_ptr(struct cache *cache)
 {
-	return (struct shared_context *)((unsigned char *)cache - ((struct shared_context *)NULL)->data);
+	return (struct shared_context *)((unsigned char *)cache -  offsetof(struct shared_context, data));
 }
 
 static inline struct shared_block *block_ptr(struct cache_entry *entry)
 {
-	return (struct shared_block *)((unsigned char *)entry - ((struct shared_block *)NULL)->data);
+	return (struct shared_block *)((unsigned char *)entry - offsetof(struct shared_block, data));
 }
 
 

src/ssl_sock.c

@@ -4205,7 +4205,7 @@ static inline void sh_ssl_sess_free_blocks(struct shared_block *first, struct sh
 /* return first block from sh_ssl_sess  */
 static inline struct shared_block *sh_ssl_sess_first_block(struct sh_ssl_sess_hdr *sh_ssl_sess)
 {
-	return (struct shared_block *)((unsigned char *)sh_ssl_sess - ((struct shared_block *)NULL)->data);
+	return (struct shared_block *)((unsigned char *)sh_ssl_sess - offsetof(struct shared_block, data));
 
 }