eliminate some static analyis warnings

** CID 73898:  Resource leak in object  (CTOR_DTOR_LEAK)
/src/filebrowser/data-cache.cpp: 24 in DirentsCache::DirentsCache()()

** CID 73903:  Uninitialized pointer field  (UNINIT_CTOR)
/src/filebrowser/data-cache.cpp: 57 in FileCacheDB::FileCacheDB()()

** CID 73898:  Resource leak in object  (CTOR_DTOR_LEAK)
/src/filebrowser/data-cache.cpp: 57 in FileCacheDB::FileCacheDB()()
Merge pull request #269 from haiwen/bugfix-6

eliminate some static analyis warnings