CookieBasedSessions: Don't parse decrypted bogus data

If the cookie contains data encrypted with a different key, the decrypted
string may still be semicolon-separated without actually containing key-value
pairs. Check that a part is a true key-value pair before trying to use its
value.

Fixes #1083
Merge pull request #1526 from provegard/issue-1083

Fix for IndexOutOfRangeException in CookieBasedSessions