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>

src/netutils.h

@@ -24,7 +24,7 @@
 #define _NETUTILS_H
 
 size_t get_sockaddr_len(struct sockaddr *addr);
-size_t get_sockaddr(char *host, char *port, struct sockaddr_storage *storage, int block);
+ssize_t get_sockaddr(char *host, char *port, struct sockaddr_storage *storage, int block);
 int set_reuseport(int socket);
 
 #endif