Bug 304015

In Clementine (http://www.clementine-player.org/) we use TagLib to read files tags.

For corrupted FLAC, TagLib hangs (corrupted files are attached in our bug report)

After looking at the code, the problem seems to be that TagLib is trying to read invalid data:
https://github.com/taglib/taglib/blob/master/taglib/flac/flacfile.cpp#L422
Something like data block with only 0 set. So |isLastBlock| is never set, and length is 0.
So TagLib try to read a 0 length block, which doesn't raise any error, and then allocates a new UnknownMetadataBlock here:
https://github.com/taglib/taglib/blob/master/taglib/flac/flacfile.cpp#L461
Then it goes to the next header block (actually what it guesses is the next header block).

Finally, it takes lot of time (i.e. time to read all the file) to realize that the FLAC file is invalid.
Afterwards, it spends lot of in the destructor deleting each block (and there is lot of blocks to delete at the end).

By the way, the destructor can be optimized by doesn't computing |size()| each time.
In my case, |size| on is linear, not constant. Maybe it is because I compiled TagLib in debug mode and that some optimizations weren't set in that case, or because the underlying data structure is a std::list, but it's actually slow down TagLib a lot.
However, in valid usecases, I think there are not so many blocks, so this may sounds superfluous. 

But IMHO a metadata block with 0 length doesn't make sense, so if we find one, probably something is wrong with this file and we should consider it as invalid.

I wrote a patch which consider a file as invalid if it finds a 0 length block.
TagLib tests are still OK with my patch, and my personal 70 flacs files (valid) give the same result with or without the patch, so I believe it doesn't break anything.

Reproducible: Always