core: fix use of uninitialised data

src/cryptonote_config.h

@@ -200,6 +200,7 @@ namespace cryptonote
     MAINNET = 0,
     TESTNET,
     STAGENET,
-    FAKECHAIN
+    FAKECHAIN,
+    UNDEFINED = 255
   };
 }

src/cryptonote_core/cryptonote_core.cpp

@@ -171,7 +171,8 @@ namespace cryptonote
               m_last_json_checkpoints_update(0),
               m_disable_dns_checkpoints(false),
               m_threadpool(tools::threadpool::getInstance()),
-              m_update_download(0)
+              m_update_download(0),
+              m_nettype(UNDEFINED)
   {
     m_checkpoints_updating.clear();
     set_cryptonote_protocol(pprotocol);