Fix check for empty hostname.

As reported by Arthur Zakirov, Gcc 7.1 complained about this with
-Wpointer-compare.

Discussion: https://www.postgresql.org/message-id/CAKNkYnybV_NFVacGbW=VspzAo3TwRJFNi+9iBob66YqQMZopwg@mail.gmail.com

src/interfaces/libpq/fe-connect.c

@@ -1097,7 +1097,7 @@ connectOptions2(PGconn *conn)
 			char	   *pwhost = conn->connhost[i].host;
 
 			if (conn->connhost[i].type == CHT_HOST_ADDRESS &&
-			conn->connhost[i].host != NULL && conn->connhost[i].host != '\0')
+				conn->connhost[i].host != NULL && conn->connhost[i].host[0] != '\0')
 				pwhost = conn->connhost[i].hostaddr;
 
 			conn->connhost[i].password =