nimble/transport; MYNEWT-737: Make ble_hci_uart_tx_char more efficient

Per pull request review, add assert() if om is NULL. Did not
modify the rest of the code in case assert() is compiled out
by NDEBUG being defined.

X-Original-Commit: 5a869e3e2e791fd83e763a02fcecb4803de6ce37

nimble/transport/uart/src/ble_hci_uart.c

@@ -338,6 +338,7 @@ ble_hci_uart_tx_char(void *arg)
                 }
                 /* NOTE: om should never be NULL! What to do? */
                 if (om == NULL) {
+                    assert(0);
                     ble_hci_uart_state.tx_type = BLE_HCI_UART_H4_NONE;
                 } else {
                     ble_hci_uart_state.tx_pkt.dptr = om->om_data;