chunked_array: fix a data race problem in the multithreaded test.
chunked_array: fix a data race problem in the multithreaded test.

dstoe/vigra,funkey/vigra,dagophil/vigra,dagophil/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,dagophil/vigra,funkey/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,funkey/vigra