sim: validate IMS private identity

Make sure that IMS private identity is a valid UTF8 string before
setting sim->impi field. Otherwise ofono may crash on dbus assert
when SIM properties are reported via org.ofono.SimManager interface.

src/sim.c

@@ -1664,7 +1664,8 @@ static void impi_read_cb(int ok, int total_length, int record,
 		return;
 	}
 
-	sim->impi = g_strndup((const char *)data + 2, data[1]);
+	if (validate_utf8_tlv(data))
+		sim->impi = g_strndup((const char *)data + 2, data[1]);
 }
 
 static void discover_apps_cb(const struct ofono_error *error,