MINOR: stktable: provide an unchecked version of stktable_data_ptr()

stktable_data_ptr() currently performs null pointer checks but most
callers don't check the result since they know by construction that
it cannot be null. This causes valid warnings when building with
-Wextra which are worth addressing since it will result in better
code. Let's provide an unguarded version of this function for use
where the check is known to be useless and untested.
diff --git a/include/proto/stick_table.h b/include/proto/stick_table.h
index 59674f6..40bb8ca 100644
--- a/include/proto/stick_table.h
+++ b/include/proto/stick_table.h
@@ -113,6 +113,14 @@
 	return PE_NONE;
 }
 
+/* return pointer for data type <type> in sticky session <ts> of table <t>, all
+ * of which must exist (otherwise use stktable_data_ptr() if unsure).
+ */
+static inline void *__stktable_data_ptr(struct stktable *t, struct stksess *ts, int type)
+{
+	return (void *)ts + t->data_ofs[type];
+}
+
 /* return pointer for data type <type> in sticky session <ts> of table <t>, or
  * NULL if either <ts> is NULL or the type is not stored.
  */
@@ -127,7 +135,7 @@
 	if (!ts)
 		return NULL;
 
-	return (void *)ts + t->data_ofs[type];
+	return __stktable_data_ptr(t, ts, type);
 }
 
 /* kill an entry if it's expired and its ref_cnt is zero */