suppress a warning

* ext/socket/raddrinfo.c (rsock_fd_family): suppress a
  sign-compare warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/socket/raddrinfo.c

@@ -525,7 +525,7 @@ rsock_fd_family(int fd)
     socklen_t sa_len = sizeof(sa);
 
     if (fd < 0 || getsockname(fd, &sa, &sa_len) != 0 ||
-        sa_len < offsetof(struct sockaddr, sa_family) + sizeof(sa.sa_family)) {
+        (size_t)sa_len < offsetof(struct sockaddr, sa_family) + sizeof(sa.sa_family)) {
 	return AF_UNSPEC;
     }
     return sa.sa_family;