nimble/audio: Modify BASS control point write access

Modify BASS control point write access in a way that
it would use proper mbuf for holding incoming data.

Fix handler check from simple bool comparison to ensuring
that it's not NULL.

nimble/host/audio/services/bass/src/ble_audio_svc_bass.c

@@ -298,6 +298,7 @@ ble_svc_audio_bass_add_source(uint8_t *data, uint16_t data_len, uint16_t conn_ha
     operation.op = BLE_SVC_AUDIO_BASS_OPERATION_ADD_SOURCE;
     operation.conn_handle = conn_handle;
 
+    offset++;
     operation.add_source.adv_addr.type = data[offset++];
     memcpy(operation.add_source.adv_addr.val, &data[offset], 6);
     offset += 6;
@@ -458,6 +459,7 @@ ble_svc_audio_bass_modify_source(uint8_t *data, uint16_t data_len, uint16_t conn
     operation.op = BLE_SVC_AUDIO_BASS_OPERATION_MODIFY_SOURCE;
     operation.conn_handle = conn_handle;
 
+    offset++;
     operation.modify_source.source_id = data[offset++];
 
     ble_svc_audio_bass_receive_state_find_by_source_id(&rcv_state,
@@ -530,7 +532,7 @@ ble_svc_audio_bass_set_broadcast_code(uint8_t *data, uint16_t data_len, uint16_t
         .type = BLE_AUDIO_EVENT_BASS_BROADCAST_CODE_SET,
     };
 
-    ev.bass_set_broadcast_code.source_id = data[0];
+    ev.bass_set_broadcast_code.source_id = data[1];
 
     ble_svc_audio_bass_receive_state_find_by_source_id(&rcv_state,
                                                        ev.bass_set_broadcast_code.source_id);
@@ -617,13 +619,17 @@ ble_svc_audio_bass_find_handler(uint8_t opcode)
 static int
 ble_svc_audio_bass_ctrl_point_write_access(struct ble_gatt_access_ctxt *ctxt, uint16_t conn_handle)
 {
+    uint8_t opcode;
+    uint16_t mbuf_len = OS_MBUF_PKTLEN(ctxt->om);
     struct ble_svc_audio_bass_ctrl_point_handler *handler;
+    uint8_t data[MYNEWT_VAL(BLE_SVC_AUDIO_BASS_METADATA_MAX_SZ)];
 
-    uint8_t opcode = ctxt->om->om_data[0];
+    os_mbuf_copydata(ctxt->om, 0, mbuf_len, data);
+    opcode = data[0];
 
     handler = ble_svc_audio_bass_find_handler(opcode);
 
-    if (!handler) {
+    if (handler == NULL) {
         return BLE_SVC_AUDIO_BASS_ERR_OPCODE_NOT_SUPPORTED;
     }
 
@@ -633,7 +639,7 @@ ble_svc_audio_bass_ctrl_point_write_access(struct ble_gatt_access_ctxt *ctxt, ui
         return BLE_ATT_ERR_WRITE_REQ_REJECTED;
     }
 
-    return handler->handler_cb(&ctxt->om->om_data[1], ctxt->om->om_len - 1, conn_handle);
+    return handler->handler_cb(data, mbuf_len, conn_handle);
 }
 
 static int