Don't permit close of receive-only channel.

Better panic on attempt to close nil channel.

From-SVN: r180437

gcc/go/gofrontend/expressions.cc

@@ -8153,6 +8153,8 @@ Builtin_call_expression::do_check_types(Gogo*)
 	{
 	  if (this->one_arg()->type()->channel_type() == NULL)
 	    this->report_error(_("argument must be channel"));
+	  else if (!this->one_arg()->type()->channel_type()->may_send())
+	    this->report_error(_("cannot close receive-only channel"));
 	}
       break;
 

libgo/runtime/go-close.c

@@ -16,6 +16,9 @@ __go_builtin_close (struct __go_channel *channel)
 {
   int i;
 
+  if (channel == NULL)
+    __go_panic_msg ("close of nil channel");
+
   i = pthread_mutex_lock (&channel->lock);
   __go_assert (i == 0);