CVE-2019-13307/imagemagick: follow-up patches

e6d26d4 introduces a memory leak by modifying AcquirePixelThreadSet
without according changes to DestroyPixelThreadSet. Fixed in e6d26d4.

These patches are quite obscure. Upstream did an additional change
which I can't really understand and might also be relevant (643921c4).
Asked for more information on the bug report.