MdeModulePkg/FileExplorerLib: Add return value check
    
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3113
    According to FAT specification, the length of file path
    should not larger than 260. When the length exceed 260,
    function FatLocateOFile will return EFI_INVALID_PARAMETER
    and the parameter FileHandle will be NULL. Then on the
    top-level function?an exception happens when the NULL
    pointer is passed and be used.
    So adding return value check after calling
    LibGetFileHandleFromMenu, if return value is not success,
    stop calling LibFindFiles.
    
    Cc: Jian J Wang <jian.j.wang@intel.com>
    Cc: Hao A Wu <hao.a.wu@intel.com>
    Cc: Dandan Bi <dandan.bi@intel.com>
    Cc: Eric Dong <eric.dong@intel.com>
    Signed-off-by: Wenyi Xie <xiewenyi2@huawei.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
    Reviewed-by: Dandan Bi <dandan.bi@intel.com>