gfio: fix crash when pushing 'start job' and no client connected

We should not really need this fix, but until the menu item
sensitivity is fixed, they are visible and we should avoid
the crash.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/gfio.c b/gfio.c
index ff8763d..51bac2a 100644
--- a/gfio.c
+++ b/gfio.c
@@ -1738,8 +1738,8 @@
 	struct gui_entry *ge = data;
 	struct gfio_client *gc = ge->client;
 
-	gtk_widget_set_sensitive(ge->button[START_JOB_BUTTON], 0);
-	fio_start_client(gc->client);
+	if (gc)
+		fio_start_client(gc->client);
 }
 
 static void file_open(GtkWidget *w, gpointer data);