added get_random_pid/1 error handling



I got folloing error:
2014-04-24 00:18:25.947 [error] @mod_pubsub_odbc:transaction_retry:4854 transaction return internal error: {'EXIT',{badarg,[{erlang,phash,[{1398,266305,946915},0],[]},{ejabberd_odbc_sup,get_random_pid,1,[{file,"src/ejabberd_odbc_sup.erl"},{line,87}]},{ejabberd_odbc,sql_call,2,[{file,"src/ejabberd_odbc.erl"},{line,144}]},{mod_pubsub_odbc,transaction_retry,4,[{file,"src/mod_pubsub_odbc.erl"},{line,4835}]},{mod_pubsub_odbc,'-unsubscribe_user/2-fun-2-',4,[{file,"src/mod_pubsub_odbc.erl"},{line,888}]},{lists,foreach,2,[{file,"lists.erl"},{line,1262}]}]}}

Don't specify zero to second argument of erlang:phash/2.
Thank you.