Initialize duration member variable in NimBLEAdvertising constructor.

m_duration member variable was not set previously which could trigger advertising to start if the
host was reset prior to the application calling start.

src/NimBLEAdvertising.cpp

@@ -63,6 +63,8 @@ NimBLEAdvertising::NimBLEAdvertising() : m_slaveItvl() {
     m_customScanResponseData         = false;
     m_scanResp                       = true;
     m_advDataSet                     = false;
+    // Set this to non-zero to prevent auto start if host reset before started by app.
+    m_duration                       = BLE_HS_FOREVER;
 
 } // NimBLEAdvertising