[WiFiScan] Allow allocation in _scanDone() to fail and prevent memory leak (espressif#10335)

* [WiFiScan] Allow allocation to fail and prevent memory leak

When there are many AP's seen during a scan, the allocation of `_scanResult` may fail.
Thus add `(std::nothrow)` to the `new` call.

Also it is possible the array was still present before allocating a new one.

* [WiFiScan] Use nullptr instead of 0

As suggested by @me-no-dev

Author: TD-er

libraries/WiFi/src/WiFiScan.cpp

@@ -48,7 +48,7 @@ uint32_t WiFiScanClass::_scanTimeout = 60000;
 uint16_t WiFiScanClass::_scanCount = 0;
 uint32_t WiFiScanClass::_scanActiveMinTime = 100;
 
-void *WiFiScanClass::_scanResult = 0;
+void *WiFiScanClass::_scanResult = nullptr;
 
 void WiFiScanClass::setScanTimeout(uint32_t ms) {
   WiFiScanClass::_scanTimeout = ms;
@@ -117,13 +117,18 @@ int16_t
  */
 void WiFiScanClass::_scanDone() {
   esp_wifi_scan_get_ap_num(&(WiFiScanClass::_scanCount));
+  if (WiFiScanClass::_scanResult) {
+    delete[] reinterpret_cast<wifi_ap_record_t *>(WiFiScanClass::_scanResult);
+    WiFiScanClass::_scanResult = nullptr;
+  }
+
   if (WiFiScanClass::_scanCount) {
-    WiFiScanClass::_scanResult = new wifi_ap_record_t[WiFiScanClass::_scanCount];
+    WiFiScanClass::_scanResult = new (std::nothrow) wifi_ap_record_t[WiFiScanClass::_scanCount];
     if (!WiFiScanClass::_scanResult) {
       WiFiScanClass::_scanCount = 0;
     } else if (esp_wifi_scan_get_ap_records(&(WiFiScanClass::_scanCount), (wifi_ap_record_t *)_scanResult) != ESP_OK) {
       delete[] reinterpret_cast<wifi_ap_record_t *>(WiFiScanClass::_scanResult);
-      WiFiScanClass::_scanResult = 0;
+      WiFiScanClass::_scanResult = nullptr;
       WiFiScanClass::_scanCount = 0;
     }
   }
@@ -176,7 +181,7 @@ void WiFiScanClass::scanDelete() {
   WiFiGenericClass::clearStatusBits(WIFI_SCAN_DONE_BIT);
   if (WiFiScanClass::_scanResult) {
     delete[] reinterpret_cast<wifi_ap_record_t *>(WiFiScanClass::_scanResult);
-    WiFiScanClass::_scanResult = 0;
+    WiFiScanClass::_scanResult = nullptr;
     WiFiScanClass::_scanCount = 0;
   }
 }