iface-modem-simple: clear ongoing connect cancellable on early errors

If the Simple.Connect() operation fails before the bearer connection
process starts (e.g. during the previous SIM-PIN checks or explicit
registration attempt), the ongoing connect cancellable is not being
properly cleared, and this would prevent additional new connection
attempts unless an explicit Simple.Disconnect() is called.

    $ sudo mmcli -m 1 --simple-connect="operator-id=21403,apn=internet"
    error: couldn't connect the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.MobileEquipment.NetworkTimeout: Network timeout'
    $ sudo mmcli -m 1 --simple-connect="operator-id=21403,apn=internet"
    error: couldn't connect the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.InProgress: Connection request forbidden: operation already in progress'
    $ sudo mmcli -m 1 --simple-connect="operator-id=21403,apn=internet"
    error: couldn't connect the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.InProgress: Connection request forbidden: operation already in progress'

Fix this, by making sure the ongoing connect cancellable is always
cleared when completing the DBus method invocation that created it.

Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/issues/169

src/mm-iface-modem-simple.c

@@ -237,10 +237,29 @@ typedef struct {
     GCancellable *cancellable;
 } ConnectionContext;
 
+static void
+cleanup_cancellation (ConnectionContext *ctx)
+{
+    Private *priv;
+
+    /* The ongoing connect cancellable and the one in the connection context
+     * must be the same, as they're set together, so if the one in the
+     * context doesn't exist, do nothing. */
+    if (!ctx->cancellable)
+        return;
+
+    /* If the ongoing connect cancellable is cancelled via the Simple.Disconnect
+     * method, it won't exist in the private struct, so don't assume they
+     * both exist. */
+    priv = get_private (ctx->self);
+    g_clear_object (&priv->ongoing_connect);
+    g_clear_object (&ctx->cancellable);
+}
+
 static void
 connection_context_free (ConnectionContext *ctx)
 {
-    g_clear_object (&ctx->cancellable);
+    cleanup_cancellation (ctx);
     g_clear_object (&ctx->properties);
     g_clear_object (&ctx->bearer);
     g_variant_unref (ctx->dictionary);
@@ -487,16 +506,6 @@ completed_if_cancelled (ConnectionContext *ctx)
     return TRUE;
 }
 
-static void
-cleanup_cancellation (ConnectionContext *ctx)
-{
-    Private *priv;
-
-    priv = get_private (ctx->self);
-    g_clear_object (&priv->ongoing_connect);
-    g_clear_object (&ctx->cancellable);
-}
-
 static gboolean
 setup_cancellation (ConnectionContext  *ctx,
                     GError            **error)