_BodyFileSource: Fix use after free in init()
_BodyFileSource: Fix use after free in init()

- init(fileURL:workQueue:dataAvailableHandler:) was using the closure
  argument for the buffer pointer outside of the closure, after
  .deallocate() was called.

parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation