don't process a corrupted gamelist_diff further

fixes #3017, assuming that the error is actually casued by the
'process_gamelist_diff failed'

As process_gamelist_diff simply returns after getting an error its quite
likeley that the gamelist is then in a  corrupted state so we stop
processing it until we get a fresh gamelist.

(cherry-picked from commit 367aec2bb2d5fc5f5acbb53842441c6190c9d452)

src/game_initialization/lobby_info.cpp

@@ -126,6 +126,17 @@ void lobby_info::process_gamelist(const config& data)
 }
 
 bool lobby_info::process_gamelist_diff(const config& data)
+{
+	if(!process_gamelist_diff_impl(data)) {
+		// the gamelist is now corrupted, stop further processing and wait for a fresh list.
+		gamelist_initialized_ = false;
+		return false;
+	}
+	else {
+		return true;
+	}
+}
+bool lobby_info::process_gamelist_diff_impl(const config& data)
 {
 	SCOPE_LB;
 	if(!gamelist_initialized_) {

src/game_initialization/lobby_info.hpp

@@ -47,6 +47,8 @@ public:
 	 */
 	bool process_gamelist_diff(const config& data);
 
+	bool process_gamelist_diff_impl(const config& data);
+
 	void sync_games_display_status();
 
 	/**
@@ -140,6 +142,10 @@ public:
 		return users_;
 	}
 
+	bool gamelist_initialized() const 
+	{
+		return gamelist_initialized_;
+	}
 private:
 	void process_userlist();
 

src/gui/dialogs/multiplayer/lobby.cpp

@@ -849,6 +849,11 @@ void mp_lobby::network_handler()
 		return;
 	}
 
+	if(gamelist_diff_update_ && !lobby_info_.gamelist_initialized()) {
+		//don't process a corrupted gamelist further to prevent crashes later.
+		return;
+	}
+
 	if(gamelist_dirty_ && !delay_gamelist_update_) {
 		if(gamelist_diff_update_) {
 			update_gamelist_diff();