Fixed unchaining of event queues

The equeue_chain function is supposed to unchain the event queue
from whatever queue it is chained to when passed a null target.
Internally, this is accomplished by just calling equeue_background
with null and letting the previously registered update function
clean up the chaining.

However, equeue_chain did not appropriately check for null, causing
it to unnecessarily allocate memory and leaving the update function
in a bad state. Fixed with a simple null check.

equeue.c

@@ -547,6 +547,11 @@ static void equeue_chain_update(void *p, int ms) {
 }
 
 void equeue_chain(equeue_t *q, equeue_t *target) {
+    if (!target) {
+        equeue_background(q, 0, 0);
+        return;
+    }
+
     struct equeue_chain_context *c = equeue_alloc(q,
             sizeof(struct equeue_chain_context));
 

tests/tests.c

@@ -546,6 +546,42 @@ void chain_test(void) {
     equeue_dispatch(&q1, 30);
 
     test_assert(touched == 6);
+
+    equeue_destroy(&q1);
+    equeue_destroy(&q2);
+}
+
+void unchain_test(void) {
+    equeue_t q1;
+    int err = equeue_create(&q1, 2048);
+    test_assert(!err);
+
+    equeue_t q2;
+    err = equeue_create(&q2, 2048);
+    test_assert(!err);
+
+    equeue_chain(&q2, &q1);
+
+    int touched = 0;
+    int id1 = equeue_call(&q1, simple_func, &touched);
+    int id2 = equeue_call(&q2, simple_func, &touched);
+    test_assert(id1 && id2);
+
+    equeue_dispatch(&q1, 0);
+    test_assert(touched == 2);
+
+    equeue_chain(&q2, 0);
+    equeue_chain(&q1, &q2);
+
+    id1 = equeue_call(&q1, simple_func, &touched);
+    id2 = equeue_call(&q2, simple_func, &touched);
+    test_assert(id1 && id2);
+
+    equeue_dispatch(&q2, 0);
+    test_assert(touched == 4);
+
+    equeue_destroy(&q1);
+    equeue_destroy(&q2);
 }
 
 // Barrage tests
@@ -660,6 +696,7 @@ int main() {
     test_run(sloth_test);
     test_run(background_test);
     test_run(chain_test);
+    test_run(unchain_test);
     test_run(multithread_test);
     test_run(simple_barrage_test, 20);
     test_run(fragmenting_barrage_test, 20);