VFS: Fix some potential .back() segfaults

Utilities/File.cpp

@@ -772,7 +772,7 @@ bool fs::create_path(const std::string& path)
 
 #ifdef _WIN32
 	// Workaround: don't call is_dir with naked drive letter
-	if (parent.size() < path.size() && parent.back() != ':' && !is_dir(parent) && !create_path(parent))
+	if (parent.size() < path.size() && (parent.empty() || (parent.back() != ':' && !is_dir(parent) && !create_path(parent))))
 #else
 	if (parent.size() < path.size() && !is_dir(parent) && !create_path(parent))
 #endif

rpcs3/Emu/vfs_config.cpp

@@ -45,7 +45,11 @@ std::string cfg_vfs::get(const std::string& _cfg, const std::string& def, std::s
 	path = fmt::replace_all(path, "$(EmulatorDir)", emu_dir);
 
 	// Check if path does not end with a delimiter
-	if (path.back() != fs::delim[0] && path.back() != fs::delim[1])
+	if (path.empty())
+	{
+		vfs_log.error("VFS config path empty (_cfg='%s', def='%s', emu_dir='%s')", _cfg, def, emu_dir);
+	}
+	else if (path.back() != fs::delim[0] && path.back() != fs::delim[1])
 	{
 		path += '/';
 	}
@@ -74,6 +78,11 @@ cfg::device_info cfg_vfs::get_device_info(const cfg::device_entry& _cfg, std::st
 			def_path = def_it->second.path;
 		}
 
+		if (info.path.empty() && def_path.empty())
+		{
+			return {};
+		}
+
 		info.path = get(info.path, def_path, emu_dir);
 		return info;
 	}

rpcs3/rpcs3qt/vfs_dialog_usb_input.cpp

@@ -11,6 +11,7 @@ vfs_dialog_usb_input::vfs_dialog_usb_input(const QString& name, const cfg::devic
 	: QDialog(parent), m_gui_settings(std::move(_gui_settings)), m_gui_save(gui::fs_dev_usb_list)
 {
 	ensure(!!info);
+	ensure(!name.isEmpty());
 	ensure(name.back() >= '0' && name.back() <= '7');
 
 	setWindowTitle(tr("Edit %0").arg(name));