BUG/MINOR: qpack: fix potential null-pointer dereference in qpack_dht_insert()

BUG/MINOR: qpack: fix potential null-pointer dereference in qpack_dht_insert()

When defragmenting the QPACK dynamic header table upfront during an
insertion, qpack_dht_defrag() can fail and return NULL if memory
allocation or re-allocation fails.

However, qpack_dht_insert() was blindly using the returned pointer
without validation, immediately leading to a null-pointer dereference
on 'dht->wrap'.

Fix this by checking if 'dht' is NULL after the defrag call and return
an error (-1).

Note that this has no impact on production yet because the QPACK dynamic
table is currently not enabled/used, so qpack_dht_insert() is never called.

Should be easily backported to all versions.