Fix memleak in OpenEXR lib [IMAGE-00]

 Direct leak of 8 byte(s) in 1 object(s) allocated from:
     #0 0x5d9c7f in operator new(unsigned long) (/home/pgrunt/src/image_flat/build_asan/iface/unit_tests/iface_unittest+0x5d9c7f)
     opencv#1 0x7fa5e5fbce7a in IlmThread_opencv::ThreadPool::ThreadPool(unsigned int) /home/pgrunt/src/image_flat/ext_libs/opencv/3rdparty/openexr/IlmThread/IlmThreadPool.cpp:758:29
     opencv#2 0x7fa5e5fbce7a in IlmThread_opencv::ThreadPool::globalThreadPool() /home/pgrunt/src/image_flat/ext_libs/opencv/3rdparty/openexr/IlmThread/IlmThreadPool.cpp:838

Taken from AcademySoftwareFoundation/openexr#412

3rdparty/openexr/IlmThread/IlmThreadPool.cpp

@@ -566,9 +566,11 @@ ThreadPool::Data::~Data()
 {
 #ifdef ILMBASE_FORCE_CXX03
     provider->finish();
+    delete provider;
 #else
     ThreadPoolProvider *p = provider.load( std::memory_order_relaxed );
     p->finish();
+    delete p;
 #endif
 }