Make sure of_hash_seed is never initialized to 0

of_hash_seed == 0 is special, as it means to OFMapTable to behave
deterministically (which is required for the tests, for example) and not
do any random per-hashtable rotating on the hash. The random rotation
improves resistance against hash collision DoS attacks, so if the random
seed happens to be 0, it would disable that rotation unintentionally,
thus it's better to get a new random seed if it is 0.