pf: improve DIOCRCLRTABLES validation

Unterminated strings in the anchor or name could cause crashes.
Validate them, and add a test case.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 1da3c0ca5b1decaa9cf55859cd134bdcd1218116)