Polish code

code issues found by Clang with
CFLAGS="-Os -Wall -Wextra -Werror=unused-const-variable -Wno-error=unused-parameter"
enabled

The details as follows:
- variable initialization issue, modified them as Clang suggested
- data type mismatch when comparing, fix them either by casting or changing
  the data type of index of iterator
- get_sockaddr() in netutils may return -1 thus changed returning type from
  size_t to ssize_t, which makes Clang happier
- recvfrom() in udprelay is identical, fix this by creating local variable
  for comparison  and assign back to buf->len when returning value is acceptable

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

libipset/bdd/write.c

@@ -541,7 +541,8 @@ ipset_node_cache_save_dot(struct cork_stream_consumer *stream,
                           struct ipset_node_cache *cache, ipset_node_id node)
 {
     struct dot_data  dot_data = {
-        0                       /* default value */
+        0,                       /* default value */
+        {NULL, 0, 0}
     };
 
     struct save_data  save_data;