MdeModulePkg/Variable: Check if there is a NV Variable Storage header prior to use...

MdeModulePkg/Variable: Check if there is a NV Variable Storage header prior to use its attributes

The Variable PEI and RuntimeDxe drivers were using the attribute 'HeaderLength' of
EFI_FIRMWARE_VOLUME_HEADER without checking if a Firmware Volume Header was existing at
the base address.
In case the Firmware Volume Header does not exist or is corrupted, the attribute 'HeaderLength'
is a non valid value that can lead to a non valid physical address when accessing produces an
access error.

Signed-off-by: oliviermartin
Reviewed-by: niruiyu
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12836 6f19259b-4bc3-4df7-8a09-765794883524