BUG/MINOR: stick-table: handle out-of-memory condition gracefully

In case `pool_alloc2()` returns NULL, propagate the condition to the
caller. This could happen when limiting the amount of memory available
for HAProxy with `-m`.

[wt: backport to 1.6 and 1.5 needed]
(cherry picked from commit ef8f4fe12d03d6e015b6e5bfa0a9d063cff83b89)
(cherry picked from commit 797c48a454fa1a7181b6b11cf93b38dd6c2a62b0)
1 file changed