Make sure account is valid when merging topics

Rolls back changes in 8893587 that created an issue with the reader when noone was logged in.
Improve check, make sure account is identical.
Improve equality check anticipating edge cases.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/2083-crash-when-account-is-nil-part-deux

Conflicts:
	WordPress/Classes/Services/ReaderTopicService.m
Merge pull request #2184 from wordpress-mobile/issues/2083-crash-when-account-is-nil-part-deux

Make sure account is valid when merging topics