Cast dport and sport to u_int before shifting them.

Cast dport and sport to u_int before shifting them.

The result of the expression is ultimately going to be put into a u_int;
cast them to u_int so that we'll be shifting unsigned values left rather
than int values, to avoid undefined behavior.

This should fix GitHub issue #681.

(cherry picked from commit cde392ab16d1de6b88f6b70dca6ddd4b83d8b525)