lib: Fix memory leak in in Link State

lib: Fix memory leak in in Link State

When using ls_stream2ted() function to parse Opaque Link State message to local
TED, in case of vertex or subnet deletion, the function return a pointer to the
deleted ls_element instead of NULL. This could lead into a potential pointer
corruption when caller try to access to the deleted ls_element.

This patch ensure that the ls_element pointer return by ls_stream2ted()
function is NULL when the message event is a delete operation for vertex and
subnet. Note that edge deletion was correctly handled.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>