More robust checking to prevent fatal IndexOutOfBoundsException

Even with the fix in the parent commit,  the X-K9mail-Identity header can
become invalid if, for example, a user creates a draft in K-9 Mail, then
edits the draft outside of K-9 Mail, then opens the draft again in K-9
Mail.

This commit assures that an invalid X-K9mail-Identity header will not
result in an IndexOutOfBoundsException.