call InitializeSections() in DEH_LoadLump()

apparently, this has been forgotten before, so DEH_PointerInit()
was not executed which meant that *some* DEHACKED patches
would not work when loaded from within a PWAD but would work
when extracted from that WAD and loaded via -deh.

fixes #421
Merge branch 'master' of https://github.com/chocolate-doom/chocolate-doom