legacy_mgcp: mgcp_test: sanitize: free msgb_ctx

Ensure that all msgb were cleaned up, then free the overall msgb_ctx, in order
to not leave any memory leaks the sanitizer build complains about.

Backport from osmo-mgw.git

Change-Id: I84e0ac7f0928f04ffddd7da18200466841589c25
diff --git a/openbsc/tests/mgcp/mgcp_test.c b/openbsc/tests/mgcp/mgcp_test.c
index 154d0f3..92099db 100644
--- a/openbsc/tests/mgcp/mgcp_test.c
+++ b/openbsc/tests/mgcp/mgcp_test.c
@@ -1208,7 +1208,7 @@
 
 int main(int argc, char **argv)
 {
-	msgb_talloc_ctx_init(NULL, 0);
+	void *msgb_ctx = msgb_talloc_ctx_init(NULL, 0);
 	osmo_init_logging(&log_info);
 
 	test_strline();
@@ -1227,6 +1227,9 @@
 	test_no_name();
 	test_osmux_cid();
 
+	OSMO_ASSERT(talloc_total_size(msgb_ctx) == 0);
+	OSMO_ASSERT(talloc_total_blocks(msgb_ctx) == 1);
+	talloc_free(msgb_ctx);
 	printf("Done\n");
 	return EXIT_SUCCESS;
 }