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/NimBLEScan.cpp

@@ -292,6 +292,11 @@ bool NimBLEScan::isScanning() {
 bool NimBLEScan::start(uint32_t duration, bool is_continue) {
     NIMBLE_LOGD(LOG_TAG, ">> start: duration=%" PRIu32, duration);
 
+    if (NimBLEDevice::isConnectionInProgress()) {
+        NIMBLE_LOGE(LOG_TAG, "Connection in progress, cannot start scan");
+        return false;
+    }
+
     // Save the duration in the case that the host is reset so we can reuse it.
     m_duration = duration;