BUG/MEDIUM: stick_table: fix crash when using tcp smp_fetch_src

BUG/MEDIUM: stick_table: fix crash when using tcp smp_fetch_src

Since the introduction of bc_src, smp_fetch_src from tcp_sample inspect
the kw argument to choose between the frontend or the backend source
address. However, for the stick tables, the argument is left to NULL.
This causes a segfault.

Fix the crash by explicitely set the kw argument to "src" to retrieve
the source address of the frontend side.

This bug was introduced by the following commit :
  7d081f02a43651d781a3a30a51ae19abdceb5683
  MINOR: tcp_samples: Add samples to get src/dst info of the backend connection

It does not need a backport as it is integrated in the current 2.4-dev
branch.

To reproduce the crash, I used the following config :

frontend fe
	bind :20080
	http-request track-sc0 src table foo
	http-request reject if { src_conn_rate(foo) gt 10 }
	use_backend h1

backend foo
	stick-table type ip size 200k expire 30s store conn_rate(60s)

backend h1
	server nginx 127.0.0.1:30080 check

This should fix the github issue #1247.