MT#55984 safeguard against possible null return

`call_get_monologue_new` is supposed to always return a full dialogue,
but an error in invocation (using a from-tag that doesn't belong to an
offer/answer) can lead to the second half being unset. Return an error
in this case.

Change-Id: I84b21ff5e5c0403fc07cae83fee206705ecff8b3

daemon/call.c

@@ -4221,6 +4221,8 @@ ok_check_tag:
 		break; // there should only be one
 		// XXX check if there's more than a one-to-one mapping here?
 	}
+	if (G_UNLIKELY(!os))
+		return -1;
 	__tags_associate(ret, os);
 	dialogue[0] = ret;
 	dialogue[1] = os;