add timeout to _run_until loop

fixes esp8266#6464

Author: johnm545

libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.cpp

@@ -437,9 +437,16 @@ int WiFiClientSecure::_run_until(unsigned target, bool blocking) {
     DEBUG_BSSL("_run_until: Not connected\n");
     return -1;
   }
+  
+  unsigned long startMillis = millis();
   for (int no_work = 0; blocking || no_work < 2;) {    
     optimistic_yield(100);
 
+    if (millis() - startMillis > 30000) {
+      DEBUG_BSSL("_run_until: Timeout\n");
+      return -1;
+    }
+    
     int state;
     state = br_ssl_engine_current_state(_eng);
     if (state & BR_SSL_CLOSED) {