BUG/MAJOR: Stick-tables crash with segfault when the key is not in the stick-table

BUG/MAJOR: Stick-tables crash with segfault when the key is not in the stick-table

When a lookup is done on a key not present in the stick-table the "st"
pointer is NULL and it is used to return the converter result, but it
is used untested with stktable_release().

This regression was introduced in 1.8.10 here:

   BUG/MEDIUM: stick-tables: Decrement ref_cnt in table_* converters
   commit d7bd88009d88dd413e01bc0baa90d6662a3d7718
   Author: Daniel Corbett <dcorbett@haproxy.com>
   Date:   Sun May 27 09:47:12 2018 -0400

Minimal conf for reproducong the problem:

   frontend test
      mode http
      stick-table type ip size 1m expire 1h store gpc0
      bind *:8080
      http-request redirect location /a if { src,in_table(test) }

The segfault is triggered using:

   curl -i http://127.0.0.1:8080/

This patch must be backported in 1.8