fix #814: signout if corrupted db detected

fix #814: signout if corrupted db detected

I don't like manual checks here https://github.com/wordpress-mobile/WordPress-Android/blob/328a6abc9a181a1a128c58521d515d51e96b3092/src/org/wordpress/android/WordPress.java#L147 but I dont know how we could automate this.

Also, I was thinking of adding constraints in the db schema (like `blogName not null`)