Add a null check for the endpoint on shutdown

src/core/lib/channel/handshaker.cc

@@ -223,6 +223,10 @@ static bool call_next_handshaker_locked(grpc_handshake_manager* mgr,
       mgr->index == mgr->count) {
     if (error == GRPC_ERROR_NONE && mgr->shutdown) {
       error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("handshaker shutdown");
+      // It is possible that the endpoint has already been destroyed by
+      // a shutdown call while this callback was sitting on the ExecCtx
+      // with no error.
+      if (mgr->args.endpoint != nullptr) {
         // TODO(roth): It is currently necessary to shutdown endpoints
         // before destroying then, even when we know that there are no
         // pending read/write callbacks.  This should be fixed, at which
@@ -236,6 +240,7 @@ static bool call_next_handshaker_locked(grpc_handshake_manager* mgr,
         gpr_free(mgr->args.read_buffer);
         mgr->args.read_buffer = nullptr;
       }
+    }
     if (grpc_handshaker_trace.enabled()) {
       gpr_log(GPR_INFO,
               "handshake_manager %p: handshaking complete -- scheduling "