thresholds: Fix buffer overflow in threshold context

thresholds: Fix buffer overflow in threshold context

th_entry is resized using ThresholdHashRealloc() every time a rule with
a threshold using by_rule tracking is added. The problem is that this is
done before the rules are reordered, so occasionally a rule with by_rule
tracking gets a higher signature number (after reordering) than the
number of th_entries allocated, causing Suricata to crash.

This commit fixes this by allocating th_entries after all the rules are
loaded and reordered.

Backtrace from core dump:

  Program terminated with signal SIGSEGV, Segmentation fault.

  #0  0x000000000051b381 in ThresholdHandlePacket (p=p@entry=0x7fb0080f3960, lookup_tsh=0x51, new_tsh=new_tsh@entry=0x7fb016c316e0, td=td@entry=0x14adedf0, sid=9800979, gid=1, pa=0x7fb0080f3b18)
      at detect-engine-threshold.c:415
  415>----                if (TIMEVAL_DIFF_SEC(p->ts, lookup_tsh->tv1) < td->seconds) {

Bug #4503.