fix: dev: Only unlink from SIEVE LRU if it is still linked

Under the overmem conditions, the header could get unlinked from the
SIEVE LRU using a different path.  This could lead to double-unlink
which causes assertion failure.  Add a guard to ISC_SIEVE_UNLINK() to
unlink only still linked headers.

Closes #5606

Merge branch '5606-fix-assertion-failure-in-overmem-cleaning' into 'main'

See merge request isc-projects/bind9!11166