Fix code potential errors.

Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Tian, Feng <feng.tian@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13852 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Bus/Ata/AtaBusDxe/AtaBus.c

@@ -464,6 +464,7 @@ UnregisterAtaDevice (
   if (BlockIo != NULL) {
     AtaDevice = ATA_DEVICE_FROM_BLOCK_IO (BlockIo);
   } else {
+    ASSERT (BlockIo2 != NULL);
     AtaDevice = ATA_DEVICE_FROM_BLOCK_IO2 (BlockIo2);
   }