BUG/MEDIUM: stick-tables: Don't forget to dec count on failure.

In stksess_new(), if we failed to allocate memory for the new stksess,
don't forget to decrement the table entry count, as nobody else will
do it for us.
An artificially high count could lead to at least purging entries while
there is no need to.

This should be backported up to 2.8.

WIP decrement current on allocation failure

src/stick_table.c

@@ -456,7 +456,8 @@ struct stksess *stksess_new(struct stktable *t, struct stktable_key *key)
 			stksess_setkey(t, ts, key);
 			stksess_setkey_shard(t, ts, key);
 		}
-	}
+	} else
+		HA_ATOMIC_DEC(&t->current);
 
 	return ts;
 }