Fix defects found by coverity (ARMmbed#1529)

Fix CID: 223399 Unchecked return value from library
Fix CID: 223400, 223401, 223404: Null pointer dereferences

Author: Arto Kinnunen

source/6LoWPAN/Thread/thread_extension_bbr.c

@@ -571,6 +571,8 @@ static int thread_extension_bbr_mlr_cb(int8_t service_id, uint8_t source_address
     (void)source_address;
     (void)source_port;
 
+    protocol_interface_info_entry_t *cur;
+    sn_coap_msg_code_e response_code = COAP_MSG_CODE_RESPONSE_CHANGED;
     uint16_t addr_len;
     uint16_t session_id;
     uint8_t *addr_data_ptr;
@@ -583,12 +585,15 @@ static int thread_extension_bbr_mlr_cb(int8_t service_id, uint8_t source_address
     tr_info("Thread BBR MLR.ntf receive");
 
     thread_pbbr_t *this = thread_border_router_find_by_service(service_id);
-    protocol_interface_info_entry_t *cur = protocol_stack_interface_info_get_by_id(this->interface_id);
-    sn_coap_msg_code_e response_code = COAP_MSG_CODE_RESPONSE_CHANGED;
-
     if (!this) {
         return -1;
     }
+
+    cur = protocol_stack_interface_info_get_by_id(this->interface_id);
+    if (!cur) {
+        return -1;
+    }
+
     ptr = payload;
 
     // Set default value if not specified in message.

source/6LoWPAN/Thread/thread_joiner_application.c

@@ -1122,7 +1122,8 @@ void thread_joiner_pending_config_activate(int8_t interface_id)
 
     if (master_key_ptr && memcmp(master_key_ptr,link_configuration->master_key,16) != 0) {
         this->configuration_ptr->key_sequence = 0;
-        thread_nvm_store_seq_counter_write(this->configuration_ptr->key_sequence);
+        // if write fails, keep going...
+        (void)thread_nvm_store_seq_counter_write(this->configuration_ptr->key_sequence);
     }
 
     tr_info("*** Activating pending configuration.");