BUG/MEDIUM: sticktables: segfault in some configuration error cases

When a stick table is tracked, and another one is used later on the
configuration, a segfault occurs.

The function "smp_create_src_stkctr" can return a NULL value, and
its value is not tested, so one other function try to dereference
a NULL pointer. This patch just add a verification of the NULL
pointer.

The problem is reproduced with this configuration:

   listen www
       mode http
       bind :12345
       tcp-request content track-sc0 src table IPv4
       http-request allow if { sc0_inc_gpc0(IPv6) gt 0 }
       server dummy 127.0.0.1:80
   backend IPv4
       stick-table type ip size 10 expire 60s store gpc0
   backend IPv6
       stick-table type ipv6 size 10 expire 60s store gpc0

Thank to kabefuna@gmail.com for the bug report.

This patch must be backported in the 1.6 and 1.5 version.
(cherry picked from commit 6fc340ff07171bb85d11d835fa4158bbdef240a0)
(cherry picked from commit 4693e2302271252044038c9be38487fb16218e5b)
diff --git a/src/session.c b/src/session.c
index 4e84803..fbd5094 100644
--- a/src/session.c
+++ b/src/session.c
@@ -2908,7 +2908,7 @@
 	smp->flags = SMP_F_VOL_TEST;
 	smp->type = SMP_T_UINT;
 	smp->data.uint = 0;
-	if (stkctr_entry(stkctr) != NULL) {
+	if (stkctr && stkctr_entry(stkctr)) {
 		void *ptr;
 
 		/* First, update gpc0_rate if it's tracked. Second, update its