fixes for coverity tool reported issues. (ARMmbed#1772)

CID: 270645, and 270646 are fixed by this pull request.

Author: deepakvenugopal

source/MAC/IEEE802_15_4/mac_mcps_sap.c

@@ -610,7 +610,7 @@ static uint8_t mac_data_interface_decrypt_packet(mac_pre_parsed_frame_t *b, mlme
             return MLME_UNAVAILABLE_KEY;
         }
 
-        if (neighbour_validation.frameCounter < b->neigh_info->FrameCounter ) {
+        if (b->neigh_info && neighbour_validation.frameCounter < b->neigh_info->FrameCounter ) {
             tr_debug("MLME_COUNTER_ERROR");
             return MLME_COUNTER_ERROR;
         }

source/MAC/IEEE802_15_4/mac_pd_sap.c

@@ -429,10 +429,9 @@ static int8_t mac_data_interface_tx_done_cb(protocol_interface_rf_mac_setup_s *r
 
         case PHY_LINK_TX_DONE_PENDING:
             mac_tx_done_state_set(rf_ptr, MAC_TX_DONE_PENDING);
-
             break;
 
-        case PHY_LINK_CCA_PREPARE:
+        default:
             break;
     }
     if (rf_ptr->fhss_api) {