BUG/MINOR: stick-table: reject negative stick counter IDs in sc-* actions

BUG/MINOR: stick-table: reject negative stick counter IDs in sc-* actions

The parsers of sc-inc-gpc, sc-inc-gpc0/1, sc-add-gpc, sc-set-gpt and
sc-set-gpt0 read the stick counter ID with strtol() into a signed int and
only check it against tune.stick-counters. A negative ID such as in
"sc-inc-gpc(0,-2)" or "sc-set-gpt0(-1)" is thus accepted, and at runtime
the actions read &s->stkctr[-2] or &sess->stkctr[-1], before the start of
the stick counters array. ASan reports a heap-buffer-overflow READ in
stkctr_entry() called from the action, and a release build may crash or
use whatever lies there as a stick-table entry pointer to update it.

The sc_* sample fetches are not affected since they convert the ID to an
unsigned int before the check, and track-sc* parses an unsigned number.

Let's also reject negative IDs. A reg-test is added.

This should be backported to all stable branches. sc-add-gpc() appeared
in 2.8 and the others are older.