Fix errors found by Coverity (#2131)

-349324 Dereference null return value
-349290 Dereference after null check

Author: Arto Kinnunen

source/6LoWPAN/ws/ws_bootstrap.c

@@ -664,10 +664,7 @@ uint16_t ws_etx_read(protocol_interface_info_entry_t *interface, addrtype_t addr
         if (!ws_neighbour || !etx_entry || etx_entry->etx_samples < 1 /*||
                 !ws_neighbour->candidate_parent*/) {
             // if RSL value is not good enough candidate parent flag is removed and device not accepted as parent
-            //tr_debug("ws_etx_read not valid parent");
-            if (etx_entry && etx_entry->etx_samples) {
-                tr_debug("ws_etx_read not valid %u RSL IN(%u), %u RSL out(%u)", ws_neighbor_class_rsl_in_get(ws_neighbour), ws_neighbour->rsl_in, ws_neighbor_class_rsl_out_get(ws_neighbour), ws_neighbour->rsl_out);
-            }
+            tr_debug("ws_etx_read not valid params");
             return 0xffff;
         }
 

source/6LoWPAN/ws/ws_common.c

@@ -415,7 +415,7 @@ void ws_common_etx_validate(protocol_interface_info_entry_t *interface, mac_neig
 {
     etx_storage_t *etx_entry = etx_storage_entry_get(interface->id, neigh->index);
 
-    if (neigh->nud_active || !neigh->trusted_device || etx_entry->etx_samples) {
+    if (neigh->nud_active || !neigh->trusted_device || !etx_entry || etx_entry->etx_samples) {
         return; //Do not trig Second NS if Active NUD already, not trusted or ETX samples already done
     }