Check validity of downloaded files

[av2k]

Describe the bug
During the download of the Autodesk installer EXE file, I stopped your installer script due to an unrelated reason.
Afterwards I re-ran the installer script. It finished, but hadn't installed Fusion at all since the Fusion installer EXE was invalid. Since there was already a software link added to Fusion, it took me a few minutes to find out that attempting to start Fusion using the desktop run link didn't do anything at all as there was simply no Fusion installation.

To Reproduce
Steps to reproduce the behavior:

1. Run the install script
2. Break/stop the script while it downloads the Fusion installer EXE
3. Re-run the installer script
4. The script doesn't install Fusion because the EXE download isn't resumed and the partially downloaded EXE is useless.

Expected behavior
The EXE is completely re-downloaded.

Additional context

Possible solution 1: Store the downloaded EXE with a suffix (e.g. installer.exe.dl) and only rename it to its real filename when CURL/wget/... return with a a success code (error code 0).

Possible solution 2: When the installer script finds downloaded EXE files, try to get the filesize of the files from the servers and compare the size to the locally stored files. If the size differs, remove the local files and redownload them.