If the data in a hashes.pkl is corrupted but still de-serialized without
errors, it will mess up the replication and gets never fixed. This
happens for example if one of the keys is a NULL byte.
This patch checks if the dict keys in hashes.pkl are valid strings and
invalidates it if not.
Closes-Bug: 1830881
Change-Id: I84b062d062ff49935feed0aee3e1963bb72eb5ea