NetworkPkg/IScsiDxe: Enhance the check for array boundary

Array 'TargetUrl' of size 255 may use index value(s) 255 and 256.
So enhance the boundary check to ensure the index is valid.

Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>