BUG/MINOR: stick-table: Always call smp_fetch_src() with a valid arg list

The sample fetch functions must always be called with a valid argument
list. When called by hand, if there is no argument to pass, empty_arg_list must
be used.

In the stick-table code, there are some calls to smp_fetch_src() with NULL as
argument list. It is changed to use empty_arg_list instead. It is not really a
bug because smp_fetch_src() does not use the argument list. But it is an API
bug.

This patch may be backported to all stable branches as a cleanup.
diff --git a/src/stick_table.c b/src/stick_table.c
index 485f6f1..548d97c 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -2247,7 +2247,7 @@
 		smp.px = NULL;
 		smp.sess = sess;
 		smp.strm = strm;
-		if (!smp_fetch_src || !smp_fetch_src(NULL, &smp, NULL, NULL))
+		if (!smp_fetch_src || !smp_fetch_src(empty_arg_list, &smp, NULL, NULL))
 			return NULL;
 
 		/* Converts into key. */
@@ -2312,7 +2312,7 @@
 	smp.px = NULL;
 	smp.sess = sess;
 	smp.strm = strm;
-	if (!smp_fetch_src || !smp_fetch_src(NULL, &smp, NULL, NULL))
+	if (!smp_fetch_src || !smp_fetch_src(empty_arg_list, &smp, NULL, NULL))
 		return NULL;
 
 	/* Converts into key. */
@@ -2844,7 +2844,7 @@
 		return 0;
 
 	/* Fetch source address in a sample. */
-	if (!smp_fetch_src || !smp_fetch_src(NULL, smp, NULL, NULL))
+	if (!smp_fetch_src || !smp_fetch_src(empty_arg_list, smp, NULL, NULL))
 		return 0;
 
 	/* Converts into key. */