fix(modem): Fixed clang-tidy warnings

* private field 'netif' is not used [clang-diagnostic-unused-private-field]
* private field 'instance' is not used [clang-diagnostic-unused-private-field]
* Call to virtual method 'FdTerminal::stop' during destruction bypasses virtual dispatch [clang-analyzer-optin.cplusplus.VirtualCall]
* unused variable 'TAG' [clang-diagnostic-unused-const-variable]
* Null pointer passed as 2nd argument to memory copy function [clang-analyzer-unix.cstring.NullArg]
* Array access (from variable 'data') results in a null pointer dereference [clang-analyzer-core.NullDereference]

components/esp_modem/src/esp_modem_netif.cpp

@@ -69,7 +69,7 @@ void Netif::receive(uint8_t *data, size_t len)
 }
 
 Netif::Netif(std::shared_ptr<DTE> e, esp_netif_t *ppp_netif) :
-    ppp_dte(std::move(e)), netif(ppp_netif)
+    ppp_dte(std::move(e))
 {
     driver.base.netif = ppp_netif;
     driver.ppp = this;