BUG/MINOR: hpack: fix harmless use of uninitialized value in hpack_dht_insert

BUG/MINOR: hpack: fix harmless use of uninitialized value in hpack_dht_insert

A warning is reported here by valgrind on first pass in hpack_dht_insert().
The cause is that the not-yet-initialized dht->head is checked in
hpack_dht_get_tail(), though the result is not used, making it have
no impact. At the very least it confuses valgrind, and maybe it makes
it harder for gcc to optimize the code path. Let's move the variable
initialization around to shut it up. Thanks to Olivier for reporting
this one.

This fix may be backported to 1.8 at least to make valgrind usage less
painful.