pf: Improve input validation
If we pass an anchor name which doesn't exist pfr_table_count() returns
-1, which leads to an overflow in mallocarray() and thus a panic.
Explicitly check that pfr_table_count() does not return an error.
Reviewed by: melifaro
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24539