[wasm] Fix uninitialized memory introduced in r330749.

Found with MSan. This was causing all the WASM MC tests to fail about
10% of the time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330764 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed