NetworkPkg: Add check logic for some variable in iSCSI driver.

v2: need to check the global variable mPrivate before using it in
the Convert AttemptConfigData To IfrNvData by Keyword function.

Add check logic for some attempt variable to enhance code in iSCSI.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>