BUG/MEDIUM: stats: prevent crash if counters not alloc with dummy one

BUG/MEDIUM: stats: prevent crash if counters not alloc with dummy one

Define a per-thread counters allocated with the greatest size of any
stat module counters. This variable is named trash_counters.

When using a proxy without allocated counters, return the trash counters
from EXTRA_COUNTERS_GET instead of a dangling pointer to prevent
segfault.

This is useful for all the proxies used internally and not
belonging to the global proxy list. As these objects does not appears on
the stat report, it does not matter to use the dummy counters.

For this fix to be functional, the extra counters are explicitly
initialized to NULL on proxy/server/listener init functions.

Most notably, the crash has already been detected with the following
vtc:
- reg-tests/lua/txn_get_priv.vtc
- reg-tests/peers/tls_basic_sync.vtc
- reg-tests/peers/tls_basic_sync_wo_stkt_backend.vtc
There is probably other parts that may be impacted (SPOE for example).

This bug was introduced in the current release and do not need to be
backported. The faulty commits are
"MINOR: ssl: count client hello for stats" and
"MINOR: ssl: add counters for ssl sessions".