BUG/MINOR: hpack-tbl: add missing NULL check after hpack_dht_defrag()

BUG/MINOR: hpack-tbl: add missing NULL check after hpack_dht_defrag()

hpack_dht_insert() has three call sites for hpack_dht_defrag(). Two of
them (lines 293 and 306) correctly check for a NULL return and bail out
with -1. The third (line 353, data-space defrag path) assigns the return
value to dht and immediately dereferences it without a NULL check.

When pool_head_hpack_tbl is exhausted, hpack_dht_alloc() returns NULL,
hpack_dht_defrag() propagates it, and line 354 dereferences NULL+0x0a
(offsetof wrap), crashing the worker with SIGSEGV.

Add a NULL check consistent with the two other call sites.

This must be backported to all stable versions.

Reported-by: Tristan (@TristanInSec)