BUILD: stick-table: make sure not to fail on task_new() during initialization

Gcc reports a potential null-deref error in the stick-table init code.
While not critical there, it's trivial to fix. This check has been
missing since 1.4 so this fix can be backported to all supported versions.

src/stick_table.c

@@ -603,6 +603,8 @@ int stktable_init(struct stktable *t)
 		t->exp_next = TICK_ETERNITY;
 		if ( t->expire ) {
 			t->exp_task = task_new(MAX_THREADS_MASK);
+			if (!t->exp_task)
+				return 0;
 			t->exp_task->process = process_table_expire;
 			t->exp_task->context = (void *)t;
 		}