If null pointer is given to g_isi_client_destroy it segfaults. Currently
this is happening when signal 15 is sent to ofonod.

---

diff --git a/gisi/client.c b/gisi/client.c
index b12dc85..a12640d 100644
--- a/gisi/client.c
+++ b/gisi/client.c
@@ -143,6 +143,8 @@ void g_isi_client_destroy(GIsiClient *client)
 {
        unsigned id;

+       if (!client)
+               return;
        g_source_remove(client->source);
        for (id = 0; id < 256; id++)
                if (client->timeout[id] > 0)