[x86] Initialize a pointer to null to fix a bug in r212602.

This should restore GCC hosts (which happen to put the bad stuff into
the pointer) and MSan, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212606 91177308-0d34-0410-b5e6-96231b3b80d8