valgrind tool reported error fix: (ARMmbed#1746)

added check for buffer.

Author: deepakvenugopal

source/MAC/IEEE802_15_4/mac_mcps_sap.c

@@ -1748,6 +1748,10 @@ static int8_t mcps_generic_packet_rebuild(protocol_interface_rf_mac_setup_s *rf_
     dev_driver_tx_buffer_s *tx_buf = &rf_ptr->dev_driver_tx_buffer;
     ccm_globals_t ccm_ptr;
 
+    if (!buffer) {
+        return -1;
+    }
+
     if (buffer->mac_header_length_with_security == 0) {
         rf_ptr->mac_tx_status.length = buffer->mac_payload_length;
         uint8_t *ptr = tx_buf->buf;