Fix error found by coverity (ARMmbed#2389)

Fix error found by coverity

CID 682646: Dereference null return value

Author: Arto Kinnunen

source/6LoWPAN/ws/ws_pae_supp.c

@@ -286,8 +286,9 @@ int8_t ws_pae_supp_nw_key_valid(protocol_interface_info_entry_t *interface_ptr,
 
     /* If border router EUI-64 received on bootstrap complete does not match to
        EUI-64 stored with keys, delete keys */
-    if (memcmp(ptk_eui_64, pae_supp->comp_br_eui_64, 8) != 0) {
-        tr_warn("Delete keys: PTK EUI-64 %s does not match to BR EUI-64 %s", tr_array(ptk_eui_64, 8), tr_array(pae_supp->comp_br_eui_64, 8));
+    if (!ptk_eui_64 || memcmp(ptk_eui_64, pae_supp->comp_br_eui_64, 8) != 0) {
+        tr_warn("Delete keys: PTK EUI-64 %s does not match to BR EUI-64 %s",
+                ptk_eui_64 ? tr_array(ptk_eui_64, 8) : "", tr_array(pae_supp->comp_br_eui_64, 8));
         sec_prot_keys_pmk_delete(&pae_supp->entry.sec_keys);
         sec_prot_keys_ptk_delete(&pae_supp->entry.sec_keys);
         sec_prot_keys_ptk_eui_64_delete(&pae_supp->entry.sec_keys);