sgsn/test: Fix msgb freeing in mocked bssgp_tx_dl_ud

Currently the mocked bssgp_tx_dl_ud function used for testing in
sgsn_test.c does not free the msg as it is done by the original
function. This leads to a msgb leak in the test.

This patch fixes the test code accordingly and uncomments the
assertion that checks for left-over msgbs.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index ae7a715..ebe12c9 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -53,6 +53,7 @@
 		   struct bssgp_dl_ud_par *dup)
 {
 	sgsn_tx_counter += 1;
+	msgb_free(msg);
 	return 0;
 }
 
@@ -1463,8 +1464,7 @@
 	printf("Done\n");
 
 	talloc_report_full(tall_bsc_ctx, stderr);
-	/* TODO: The following check currently fails */
-	/* OSMO_ASSERT(talloc_total_blocks(tall_msgb_ctx) == 1); */
+	OSMO_ASSERT(talloc_total_blocks(tall_msgb_ctx) == 1);
 	return 0;
 }