MdeModulePkg/DxeIplPeim: Initialize pointer PageMapLevel5Entry

MdeModulePkg/DxeIplPeim: Initialize pointer PageMapLevel5Entry

Initialize PageMapLevel5Entry at the beginning of the function.

This commit will fix a GCC 4.8.5 build failure introduced by commit
b3527dedc3951f061c5a73cb4fb2b0f95f47e08b.

OvmfPkg build failure wtih gcc 4.8.5 still exists at latest edk2 version.
The commit 46f8a6891606746ca8b1e684ac379ce271306dc0 seems not to fix
the build failure completely.

Cc: Dandan Bi <dandan.bi@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>