tail: Fix heap overflow in -F case.

The number of events we track can vary over time, but we only allocate
enough space for the exact number of events we are tracking when we
first begin, resulting in a trivially reproducable heap overflow. Fix
this by allocating enough space for the greatest possible number of
events (two per file) and clean up the code a bit.

Also add a test case which triggers the aforementioned heap overflow,
although we don't currently have a way to detect it.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D42839