ShellPkg/UefiShellCommandLib: suppress incorrect gcc warning
    
    `Dupes` is used only if `Duplicates != NULL` and function is left if
    allocation of memory for `Dupes` fails, so it can't be used
    uninitialized.
    
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3228
    Cc: Ray Ni <ray.ni@intel.com>
    Cc: Zhichao Gao <zhichao.gao@intel.com>
    Signed-off-by: Sergei Dmitrouk <sergei@posteo.net>
    Message-Id: <20210511225616.5942-2-sergei@posteo.net>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>