use nullptr instead of 0

Author: d-a-v

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -58,7 +58,7 @@ class ClientContext
             tcp_err(_pcb, NULL);
             tcp_poll(_pcb, NULL, 0);
             tcp_abort(_pcb);
-            _pcb = 0;
+            _pcb = nullptr;
         }
         return ERR_ABRT;
     }
@@ -79,7 +79,7 @@ class ClientContext
                 tcp_abort(_pcb);
                 err = ERR_ABRT;
             }
-            _pcb = 0;
+            _pcb = nullptr;
         }
         return err;
     }
@@ -536,7 +536,7 @@ class ClientContext
         tcp_sent(_pcb, NULL);
         tcp_recv(_pcb, NULL);
         tcp_err(_pcb, NULL);
-        _pcb = NULL;
+        _pcb = nullptr;
         _notify_error();
     }