metadata: Be more resistive to broken journal files

In shared NFS homedir case with multiple clients writing to the same
mmaped journal file data can get easily corrupted. The daemon iterates
over a journal file on flush taking in account variable entry size and
advances according to the data read.

However in certain case invalid data are read making us to jump out of
bounds. In case of zero entry size we would stand at the same place
leading to infinite loop.

This patch checks if the indicated entry size is at least the size of
the structure we're getting the size from (it's a first element) and breaks
the iteration cycle if it's not. This may lead to partial data loss on flush
as we don't process the rest of the journal file. Old data from existing
tree file will be preserved of course, only few recent changes would get lost.

https://bugzilla.gnome.org/show_bug.cgi?id=637095

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