ECW: fix crash in GDALDeregister_ECW() with ECW SDK 5 called from GDALDestroy() (#5214)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26363 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx