cd and ls crash when passed a non-directory link

从 Cmdline.java 中 stack trace 所标识的 cd 和 ls 入口点开始，然后使用一个非目录链接或文件，从 Interpreter 重现这两次调用。跟踪每个路径的验证方式，并确认对非目录链接的预期处理；当两个命令都不再崩溃且报告的案例表现一致时，即表示完成。

    Uh oh! You've found an error in Core.
    This happened while running your code, so you may be able to find a workaround, but is ultimately an issue in Core.
    The following code caused the error:
    ls('file.zip')
    on or around Interpreter:1.
    Please report this to the developers, and be sure to include the version numbers:
    Server version: SHELL;
    MethodScript version: 3.3.4;
    Loaded extensions and versions:
    Core (3.3.4)
    Here's the stacktrace:
    java.lang.NullPointerException
            at com.laytonsmith.core.functions.Cmdline$ls.exec(Cmdline.java:1699)
            at com.laytonsmith.core.Script.eval(Script.java:409)
            at com.laytonsmith.core.MethodScriptCompiler.execute(MethodScriptCompiler.java:2818)
            at com.laytonsmith.tools.Interpreter$4.run(Interpreter.java:832)
            at java.lang.Thread.run(Thread.java:748)
    
    Uh oh! You've found an error in Core.
    This happened while running your code, so you may be able to find a workaround, but is ultimately an issue in Core.
    The following code caused the error:
    cd('file.zip')
    on or around Interpreter:1.
    Please report this to the developers, and be sure to include the version numbers:
    Server version: SHELL;
    MethodScript version: 3.3.4;
    Loaded extensions and versions:
    Core (3.3.4)
    Here's the stacktrace:
    java.lang.IllegalArgumentException: File provided to SetRootFolder must be a folder, not a file. (.\Downloads\2656-netstat-ipv6.PID.zip was found.)
            at com.laytonsmith.core.environments.GlobalEnv.SetRootFolder(GlobalEnv.java:209)
            at com.laytonsmith.core.functions.Cmdline$cd.exec(Cmdline.java:1648)
            at com.laytonsmith.core.Script.eval(Script.java:409)
            at com.laytonsmith.core.MethodScriptCompiler.execute(MethodScriptCompiler.java:2818)
            at com.laytonsmith.tools.Interpreter$4.run(Interpreter.java:832)
            at java.lang.Thread.run(Thread.java:748)