graph: add failing test for invalid node in graph_sort_successors

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: add test for invalid plugin successors

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
graph: fix GraphSuccessorSorter SIGSEV

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>