Disconnection during download init can cause a crash

[Big Muscle]

a) When download is being initialized (in DownloadManager::checkDownloads) and connection is disconnected at this time, DC++ can crash.
Patch is here: http://www.home.karneval.cz/01027053/tmp/DownloadManager.patch

b) When MerkleCheckOutputStream is created and no datas come yet, DC++ will crash if connection if disconnected.
Patch is here: http://www.home.karneval.cz/01027053/tmp/MerkleCheckOutputStream.patch

Both of these bugs could be exploited to crash DC++ remotely.

[Jacek Sieka (arnetheduck)]

applied a
b is better fixed at the source (merkletree.h)

one bug per patch please...