NetworkPkg/DxeNetLib: Change the order of conditions in IF statement

NetworkPkg/DxeNetLib: Change the order of conditions in IF statement

The condition, NET_HEADSPACE(&(Nbuf->BlockOp[Index])) < Len, is
meaningless if Index = 0. So checking 'Index != 0' should be
performed first in the if statement.

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