lib: graph: fix deletions

lib: graph: fix deletions

Iterating over an array while deleting items needs to consider
interactions between the iteration position and deletion.  The previous
code completely ignored that problem, leading to memleaks (graph_delete
skipping half of the nodes) and dangling pointers (if parallel edges
exist in graph_remove_edge).

Iterating backwards is safe and reduces "move to fill hole" overhead in
deletion.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Cc: Quentin Young <qlyoung@cumulusnetworks.com>