lapd: Set the tx_hist to NULL after freeing it

Make detecting use after free of the tx_hist easy and set the
variable to NULL after talloc_free has been freed.

src/gsm/lapd_core.c

@@ -327,6 +327,7 @@ void lapd_dl_exit(struct lapd_datalink *dl)
 	lapd_dl_reset(dl);
 	/* free history buffer list */
 	talloc_free(dl->tx_hist);
+	dl->tx_hist = NULL;
 }
 
 /*! \brief Set the \ref lapdm_mode of a LAPDm entity */