sdl2: add assert to make coverity happy

There is a loop a few lines up counting consoles and setting
sdl2_num_outputs accordingly, so con ptr can't be NULL there.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Message-id: 20170621122234.12751-1-kraxel@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/sdl2.c

@@ -804,6 +804,7 @@ void sdl_display_init(DisplayState *ds, int full_screen, int no_frame)
     sdl2_console = g_new0(struct sdl2_console, sdl2_num_outputs);
     for (i = 0; i < sdl2_num_outputs; i++) {
         QemuConsole *con = qemu_console_lookup_by_index(i);
+        assert(con != NULL);
         if (!qemu_console_is_graphic(con)) {
             sdl2_console[i].hidden = true;
         }