#1638 split archives not unpacked/tested in 7-zip

[murkymark]

1.
Having a split archive of RAR files named ".001"+".002" (old style), It doesn't detect the compression and wants to join the files. This is wrong in two ways because the resulting file is neither valid nor uncompressed then.
Normally 7-zip can detect the compression correctly even for wrong file name extensions.

2.
Having a password protected 7z split archive, and typing in the wrong password upon decompression, 7-zip doesn't inform the user and just joins the files to an extra single 7z file without asking. This is probably not what the user wants. Testing falsely says "successful" despite the wrong password.
Just joining files is no decompression!

(I always wondered why the bad design joice was made to just split a 7zip archive without any individual checksums and extra file headers like RAR does)