check ClientContext's this and _pcb once out of the ::connect() delay

fix #4078

Author: d-a-v

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -131,6 +131,11 @@ class ClientContext
         _op_start_time = millis();
         // This delay will be interrupted by esp_schedule in the connect callback
         delay(_timeout_ms);
+        // WiFi may have vanished during the delay (#4078)
+        if (!this || !_pcb) {
+            DEBUGV(":vnsh\r\n");
+            return 0;
+        }
         _connect_pending = 0;
         if (state() != ESTABLISHED) {
             abort();