Added NULL checks in uh_keydup, mysql_users_fetch, mysql_users_add,  uh_cmpfun, uh_hfun and uh_keyfree

Added NULL checks in uh_keydup(), mysql_users_fetch(),
mysql_users_add(),  uh_cmpfun(), uh_hfun() and uh_keyfree()
Added NULL checks in uh_keydup, mysql_users_fetch, mysql_users_add,  uh_cmpfun, uh_hfun and uh_keyfree

Added NULL checks in uh_keydup(), mysql_users_fetch(),
mysql_users_add(),  uh_cmpfun, uh_hfun() and uh_keyfree()