Don't copy a potentially-uninitialized variable.

Based on review discussion of r158638 with Chandler Carruth, Tobias von Koch, and Duncan Sands and a -Wmaybe-uninitialized warning from GCC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158685 91177308-0d34-0410-b5e6-96231b3b80d8