p11: Fix two instances of -Wmaybe-uninitialized in p11_child_openssl.c
    
    If uri_str was passed to the p11_child and parsing the URI failed, then
    modules would be uninitialized, but freed in the done handler with
    p11_kit_modules_finalize_and_release()
    
    Also, another warning is suppressed by setting the 's' variable to zero.
    While it cannot happen that the variable will be uninitialized, we
    should help the compiler by setting a value explicitly.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>