[poppler] Poppler 0.52.0 released

[Albert Astals Cid]

Available from
http://poppler.freedesktop.org/poppler-0.52.0.tar.xz

Changes against 0.51.0 include:
core:
 * Fix assert on reading some OCGs. Bug #99768
 * Properly initialize some RichMedia variables in corner cases. Bug #99767

qt4:
 * optcontent structure was leaking the headers items. Bug #99449
 * Cleanup objects in tests to fix memory leaks. Bug #99449

qt5:
 * optcontent structure was leaking the headers items. Bug #99449
 * Cleanup objects in tests to fix memory leaks. Bug #99449

utils:
 * pdftocairo.1: Fix typo

Testing, patches and bug reports welcome.

Cheers,
  Albert