Safer check that Uthernet is active.

Unfortunately we cannot force a hot-activation as this will cause SegFaults in the TFE code.
It does not check if the pcap handle is good, but relies on the initialisation to have succeeded.

Unfortunately, visual feedback on the settings is lost.

Signed-off-by: Andrea Odetti <mariofutire@gmail.com>

source/frontends/sdl/imgui/settingshelper.cpp

@@ -198,8 +198,7 @@ namespace sa2
 
   void saveTFEEnabled(const int enabled)
   {
-    tfe_enabled = enabled;
-    REGSAVE(TEXT(REGVALUE_UTHERNET_ACTIVE), tfe_enabled);
+    REGSAVE(TEXT(REGVALUE_UTHERNET_ACTIVE), enabled);
   }
 
   void changeBreakpoint(const DWORD nAddress, const bool enableAndSet)

source/linux/network/uthernet2.cpp

@@ -1015,7 +1015,13 @@ void registerUthernet2()
   slirp.reset();
   slirp = std::make_shared<SlirpNet>();
 #else
+  const int check = tfe_enabled;
   tfe_init();
+  if (tfe_enabled != check)
+  {
+    // tfe initialisation failed
+    return;
+  }
 #endif
   dataAddress = 0;
   initialise();