Sync patches r15544 and r15545 from main trunk.
1. Remove the assertion which may cause IsDevicePathValid() hang in DEBUG build.
2. Remove the RT attribute for HDDP variable and validate the variable content before using it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/branches/UDK2014/MdePkg@15688 6f19259b-4bc3-4df7-8a09-765794883524