Rewrite pool file, if pool file is zero-length

[raoulbhatia]

To my understanding, https://github.com/backuppc/rsync-bpc/blob/master/bpc_sysCalls.c#L784
has 2 reasons for rewriting a file

a) the file was removed from the pool
b) the file was marked for deletion and can be re-used.

I suggest to also rewrite if the pool file is zero-length. However, I am not able to understand the source code well enough to judge if chains / handling md5 collisions might play a role here.

Related to issues mentioned in backuppc/backuppc#412

Thanks,
Raoul