Fix minor BearSSL API issues (#4901)

Fixes #4882 and updates GitHub certificate fingerprint to the current one
in BearSSL_Validation example.

When setting a authentication mode or stopping, clear all others out in case
the object is being re-used.

Add in a yield during the SSL handshake to allow a graceful timeout and not
a WDT error when the remote server hiccups.  Thanks to @Jeroen88 for
finding and testing this.

libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.cpp

@@ -79,6 +79,7 @@ void WiFiClientSecure::_clearAuthenticationSettings() {
   _use_self_signed = false;
   _knownkey = nullptr;
   _sk = nullptr;
+  _ta = nullptr;
 }
 
 
@@ -177,6 +178,7 @@ void WiFiClientSecure::stop() {
     _client->abort();
   }
   WiFiClient::stop();
+  _clearAuthenticationSettings();
   _freeSSL();
 }
 
@@ -510,6 +512,7 @@ bool WiFiClientSecure::_wait_for_handshake() {
     if (br_ssl_engine_current_state(_eng) & BR_SSL_SENDAPP) {
       _handshake_done = true;
     }
+    optimistic_yield(1000);
   }
   return _handshake_done;
 }