io: Fix qio_channel_socket_close() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qio_channel_socket_close() passes @errp first to
socket_listen_cleanup(), and then, if closesocket() fails, to
error_setg_errno().  If socket_listen_cleanup() failed, this will trip
the assertion in error_setv().

Fix by ignoring a second error.

Fixes: 73564c40
Cc: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200422130719.28225-11-armbru@redhat.com>

io/channel-socket.c

@@ -704,6 +704,7 @@ qio_channel_socket_close(QIOChannel *ioc,
 {
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
     int rc = 0;
+    Error *err = NULL;
 
     if (sioc->fd != -1) {
 #ifdef WIN32
@@ -715,8 +716,8 @@ qio_channel_socket_close(QIOChannel *ioc,
 
         if (closesocket(sioc->fd) < 0) {
             sioc->fd = -1;
-            error_setg_errno(errp, errno,
-                             "Unable to close socket");
+            error_setg_errno(&err, errno, "Unable to close socket");
+            error_propagate(errp, err);
             return -1;
         }
         sioc->fd = -1;