fix: usr: Fix NULL Pointer Dereference in QP-trie Cache add()

When RRSIG(rdtype) was independently cached before the RDATA for the
rdtype itself, named would crash on the subsequent query for the RDATA
itself.  This has been fixed.

ISC would like to thank Vitaly Simonovich for bringing this
vulnerability to our attention.

Closes #5738

Merge branch '5738-null-pointer-dereference-in-qpcache-add' into 'main'

See merge request isc-projects/bind9!11539