abis_test: sanitize: fix msgb memleak

Helps fix sanitizer build on debian 9.

Change-Id: I69750d9cd2b8f30bfc6d2dfd9e62576e46989fab
diff --git a/tests/abis/abis_test.c b/tests/abis/abis_test.c
index a631712..c0605c2 100644
--- a/tests/abis/abis_test.c
+++ b/tests/abis/abis_test.c
@@ -193,6 +193,8 @@
 	/* check multiple, chained SW-descr: */
 	chk_raw("half", chain, sizeof(chain) / 2);
 	chk_raw("full", chain, sizeof(chain));
+
+	msgb_free(msg);
 }
 
 int main(int argc, char **argv)