libmm-glib: Initialize result array

In mm_modem_messaging_get_supported_storages(), initialize array to avoid a
potential crash with an uninitialized value, and a build failure due to

mm-modem-messaging.c:168:24: error: 'array' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
     *n_storages = array->len;

https://bugzilla.gnome.org/show_bug.cgi?id=729267

libmm-glib/mm-modem-messaging.c

@@ -155,7 +155,7 @@ mm_modem_messaging_get_supported_storages (MMModemMessaging *self,
                                            MMSmsStorage **storages,
                                            guint *n_storages)
 {
-    GArray *array;
+    GArray *array = NULL;
 
     g_return_val_if_fail (MM_IS_MODEM_MESSAGING (self), FALSE);
     g_return_val_if_fail (storages != NULL, FALSE);