Fix endless loop when calling scan start from scan end callback.

When attempting to connect and the scanner is running `NimBLEScan::stop` is called to stop it
which then calls the `onScanEnded` callback. If the app then starts the scan again in this
callback an endless loop will be created.

This change prevents the endless loop by setting a flag in `NimBLEDevice` that is checked before
starting a scan while a client is trying to connect.

* Adds `NimBLEDevice::setConnectionInProgress` and `NimBLEDevice::isConnectionInProgress` functions.

src/NimBLEDevice.h

@@ -183,6 +183,8 @@ class NimBLEDevice {
     static bool          isBonded(const NimBLEAddress& address);
     static bool          deleteAllBonds();
     static NimBLEAddress getBondedAddress(int index);
+    static void          setConnectionInProgress(bool inProgress);
+    static bool          isConnectionInProgress();
 # endif
 
   private:
@@ -194,6 +196,10 @@ class NimBLEDevice {
     static uint8_t                    m_ownAddrType;
     static std::vector<NimBLEAddress> m_whiteList;
 
+# if defined(CONFIG_BT_NIMBLE_ROLE_CENTRAL) || defined(CONFIG_BT_NIMBLE_ROLE_PERIPHERAL)
+    static bool m_connectionInProgress;
+# endif
+
 # if defined(CONFIG_BT_NIMBLE_ROLE_OBSERVER)
     static NimBLEScan* m_pScan;
 # endif