objc-arc: captured block variable accessed in its block literal

initializer need be null initialized before initializer takes
hold, just like any other initialized retainable object pointer. 
// rdar://11016025


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158738 91177308-0d34-0410-b5e6-96231b3b80d8