added error handling in mod_pubsub_odbc.

I got badmatch error in mod_pubsub_odbc.

2014-04-23 00:00:01 =ERROR REPORT====
Error in process <0.1866.4> on node 'ejabberd@localhost' with exit value: {{badmatch,{error,{xmlel,<<5 bytes>>,[{<<4 bytes>>,<<3 bytes>>},{<<4 bytes>>,<<4 bytes>>}],[{xmlel,<<21 bytes>>,[{<<5 bytes>>,<<35 bytes>>}],[]}]}}},[{mod_pubsub_odbc,'-unsubscribe_user/2-fun-1-',4,[{file,"src/mod_pubsub_odbc.erl"},{line,888}]},{lists,foreach,2,[{file,"lists.erl"},{line,1262}]}]}

I fixed so that we don't get crash.