fixed invalid read in cRecordingsCache::gc()

valgrind warning:
==32616== Invalid read of size 4
==32616==    at 0x41FFBE5: std::_Rb_tree_increment(std::_Rb_tree_node_base*) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.16)
==32616==    by 0x4CF3FD9: cRecordingsCache::gc() (stl_tree.h:196)
==32616==    by 0x4CFF825: cXVDRServer::Action() (xvdrserver.c:332)
==32616==    by 0x8154FF3: cThread::StartThread(cThread*) (thread.c:262)
==32616==    by 0x40B0D4B: start_thread (pthread_create.c:308)
==32616==    by 0x43C7BAD: clone (clone.S:130)
==32616==  Address 0xae479c4 is 12 bytes inside a block of size 40 free'd
==32616==    at 0x402ACFC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==32616==    by 0x4CF3FC9: cRecordingsCache::gc() (new_allocator.h:98)
==32616==    by 0x4CFF825: cXVDRServer::Action() (xvdrserver.c:332)
==32616==    by 0x8154FF3: cThread::StartThread(cThread*) (thread.c:262)
==32616==    by 0x40B0D4B: start_thread (pthread_create.c:308)
==32616==    by 0x43C7BAD: clone (clone.S:130)