Block Size 0 doesn't always mean an invalid flac file, so ignore rather than fail.

[tweedj]

I've found that scanning my Flac library there are some albums that fail with audio_metadata. Even though these files pass a 'flac -t FILE' they fail to extract with audio_metadata.
I've found that some of these files fail because of a ZERO size in a block, which is caught and triggers an exception.
While this is probably wanted behaviour for the STREAMINFO block, it seems a bit over zealous for blocks like, PADDING and SEEKTABLE.

So rather than call it a bug, I'll ask for a feature change. It requires a 1 (or2) line change in flac.py.

E.g. (Where flay.py in Downloads is the GitHub version)

[2041]$ diff  flac.py ~/Downloads/flac.py 
456,457c456,457
< 		if ( block_size == 0 ) and ( block_type == FLACMetadataBlockType.STREAMINFO ) :
< 			raise FormatError(f"FLAC metadata block {block_type}; size must be greater than 0.")
---
> 		if block_size == 0:
> 			raise FormatError("FLAC metadata block size must be greater than 0.")