polkit_driver: fix possible segfault
    
    The changes in commit c7542573 introduced possible segfault. Looking
    deeper into the code and the original code before the patch series were
    applied I think that we should report error for each function failure
    and also we shouldn't call some of the function twice.
    
    Found by coverity.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>