p2DAImporter::Open: ensure proper termination if we get a wierd default

coverity 1004324
actor: plugged two iwd2/gemrb slot conversion holes

coverity 1004425 & 1004426
fixed the wrong offset of DL_* constants

the last one could oob crash (cov 1004427) though nothing changes ingame
as all the levels still contain the same resrefs (dl_main)
Actor::GetLevelInClass: removed potential crash

if a mod added enough classes; cov 1004428
Projectile::LineTarget: plugged leak

cov 1004446
handle_overlay: leak 1004447
AREImporter::GetMap: removed leaks on fatal error path

coverity 1004451
Interface::LoadGemRBINI: terminating nulls ...
remove some useless null checks

more coverity fixes