libdiff: Detect and recover from file truncation.

If a memory-mapped file is truncated before we get to the end, the
atomizer may catch SIGBUS. Detect that, reduce the input length to
what we were actually able to read, and set a flag so the caller can
take further action (e.g. warn the user and / or start over).

Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45217