Possible fix for random pager crashes.

Possible fix for random pager crashes.

The crashes have been observed:
- in both local and remote mailboxes
- with/without notmuch
- always are some time has passed
- always have an invalid header pointer

Scenario:
- The Context holds an array of HEADER pointers.
- The pager is given a HEADER to display.
- Time passes and the mailbox changes.

mx_check_mailbox() adds new mails to the end of the Context's array,
calling mx_alloc_memory() if necessary.  The Context's totals are
updated by mx_update_context().

The only way a HEADER pointer could become invalid is if they email were
deleted/moved.  If we detect this has happened, we leave the pager.