sercomm_test: sanitize: fix msgb mem leak

This should fix the last current remaining sanitizer build failure in
libosmocore regression tests.

Helps fix sanitizer build on debian 9.

Change-Id: I4d6dd7f4348675bc77d4df5a7a0ce41f12d4a043
diff --git a/tests/sercomm/sercomm_test.c b/tests/sercomm/sercomm_test.c
index f4f1c22..058c9eb 100644
--- a/tests/sercomm/sercomm_test.c
+++ b/tests/sercomm/sercomm_test.c
@@ -47,6 +47,7 @@
 static void dlci_rx_cb(struct osmo_sercomm_inst *sercomm, uint8_t dlci, struct msgb *msg)
 {
 	printf("%s(): %s\n", __func__, msgb_hexdump(msg));
+	msgb_free(msg);
 }
 
 static struct msgb *create_mahlzeit_msg(void)