[balsa] address-book-extern: Add guards

[Peter Bloomfield <peterb src gnome org>]

commit 72ee44273a3d8aa33077267cc378a2560b765625
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Wed Jun 5 19:31:43 2019 -0400

    address-book-extern: Add guards

 libbalsa/address-book-extern.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)
---
diff --git a/libbalsa/address-book-extern.c b/libbalsa/address-book-extern.c
index 2e8583ec7..9eef5dfbf 100644
--- a/libbalsa/address-book-extern.c
+++ b/libbalsa/address-book-extern.c
@@ -392,12 +392,16 @@ libbalsa_address_book_externq_alias_complete(LibBalsaAddressBook * ab,
 const gchar *
 libbalsa_address_book_externq_get_load(LibBalsaAddressBookExternq * ab_externq)
 {
+    g_return_val_if_fail(LIBBALSA_IS_ADDRESS_BOOK_EXTERNQ(ab_externq), NULL);
+
     return ab_externq->load;
 }
 
 const gchar *
 libbalsa_address_book_externq_get_save(LibBalsaAddressBookExternq * ab_externq)
 {
+    g_return_val_if_fail(LIBBALSA_IS_ADDRESS_BOOK_EXTERNQ(ab_externq), NULL);
+
     return ab_externq->save;
 }
 
@@ -407,16 +411,20 @@ libbalsa_address_book_externq_get_save(LibBalsaAddressBookExternq * ab_externq)
 
 void
 libbalsa_address_book_externq_set_load(LibBalsaAddressBookExternq * ab_externq,
-                                      const gchar               * load)
+                                       const gchar                * load)
 {
+    g_return_if_fail(LIBBALSA_IS_ADDRESS_BOOK_EXTERNQ(ab_externq));
+
     g_free(ab_externq->load);
     ab_externq->load = g_strdup(load);
 }
 
 void
 libbalsa_address_book_externq_set_save(LibBalsaAddressBookExternq * ab_externq,
-                                      const gchar               * save)
+                                       const gchar                * save)
 {
+    g_return_if_fail(LIBBALSA_IS_ADDRESS_BOOK_EXTERNQ(ab_externq));
+
     g_free(ab_externq->save);
     ab_externq->save = g_strdup(save);
 }