BUG/MINOR: stick_table: Prevent conn_cur from underflowing

BUG/MINOR: stick_table: Prevent conn_cur from underflowing

When using the peers feature a race condition could prevent
a connection from being properly counted. When this connection
exits it is being "uncounted" nonetheless, leading to a possible
underflow (-1) of the conn_curr stick table entry in the following
scenario :

  - Connect to peer A     (A=1, B=0)
  - Peer A sends 1 to B   (A=1, B=1)
  - Kill connection to A  (A=0, B=1)
  - Connect to peer B     (A=0, B=2)
  - Peer A sends 0 to B   (A=0, B=0)
  - Peer B sends 0/2 to A (A=?, B=0)
  - Kill connection to B  (A=?, B=-1)
  - Peer B sends -1 to A  (A=-1, B=-1)

This fix may be backported to all supported branches.