BSSMAP: add assert to reset resending

There seems to be no way for this function to be called with NULL parameter despite unreproducible crash observed in the
past. Let's add assert to show this explicitly.

Fixes: OS#5551
Change-Id: I235bdd42ea82e7b5a1a40f437ca34c49ad239c48

src/osmo-bsc/bssmap_reset.c

@@ -245,6 +245,8 @@ bool bssmap_reset_is_conn_ready(const struct bssmap_reset *bssmap_reset)
 
 void bssmap_reset_resend_reset(struct bssmap_reset *bssmap_reset)
 {
+	OSMO_ASSERT(bssmap_reset);
+
 	/* Immediately (1ms) kick off reset sending mechanism */
 	osmo_fsm_inst_state_chg_ms(bssmap_reset->fi, BSSMAP_RESET_ST_DISC, 1, 0);
 }