Fix NULL read from RPL header addition

There is a possibility that the function rpl_data_exthdr_provider_hbh
was called with NULL Neighbor info causing an NULL pointer read
when adding headers to routed packets

Author: Mika Tervonen

source/RPL/rpl_upward.c

@@ -533,6 +533,9 @@ bool rpl_neighbour_update_dtsn(rpl_neighbour_t *neighbour, uint8_t dtsn)
 
 rpl_instance_t *rpl_neighbour_instance(const rpl_neighbour_t *neighbour)
 {
+    if (!neighbour || !neighbour->dodag_version || !neighbour->dodag_version->dodag) {
+        return NULL;
+    }
     return neighbour->dodag_version->dodag->instance;
 }