Fix encoding problems with 7z archives, v2.

From: Benoit Pierre <benoit.pierre@gmail.com>

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1044 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Only decode 7z output if encoding is actually known.

Since Linux 7z does not support the -scc switch, the output encoding
is locale-dependant, which is already handled internally
without the need to try and manually convert encodings in
SevenZipArchive.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1045 ef8490bc-f317-4725-81ed-e95a9aca1bcf