MdeModulePkg/DisplayEngine: Remove useless NULL ptr check for NewPos

MdeModulePkg/DisplayEngine: Remove useless NULL ptr check for NewPos

In function UiDisplayMenu, the NewPos ptr which used to point to the
highlight menu entry. It will always point to the menu entry which
need to be highlighted or the gMenuOption menu if the highlight menu
is not found.
So we can remove the NULL ptr check for NewPos in this function.
And add the ASSERT code to avoid if any false positive reports
of NULL pointer dereference issue raised from static analysis.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>