Incomplete trees can be added in hash database

Steps to reproduce:

1. Start hashing a large file
2. Exit the application while the hashing is still in progress
3. Open HashIndex.xml and you'll find the file from there

Looks like the application isn't happy with the saved data as it begins to hash the file again on next startup. I didn't investigate why that was the case, but I'd say that the current behavior is quite risky if the code is being modified in future (and the invalid entry will not get rehashed, which is what happened with AirDC++).

Possibly the application could behave differently if "stop" is true: https://sourceforge.net/p/dcplusplus/code/ci/default/tree/dcpp/HashManager.cpp#l683