lib: Use g_steal_pointer() more

Generated by the following spatch:

  @@
  expression a, b;
  @@

  + b = g_steal_pointer(&a);
  - b = a;
    ... when != a
  - a = NULL;

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>