[BUG] Failed download causes crash

[pryrt]

@ozone10 had a crash report:

CollectionInterface plugin will crash notepad++ after dismissing message box with error that connection is blocked.

with further details:

here is quick "static" analyses

NppPlugin-CollectionInterface/src/Classes/CollectionInterfaceClass.cpp

Line 244 in 207a0d3

std::vector<char> vcThemeJSON = downloadFileInMemory(L"https://raw.githubusercontent.com/notepad-plus-plus/nppThemes/master/themes/.toc.json");

You don't check if downloadFileInMemory returns empty vector which will I think cause issue for next 2 lines 245-246.