multi_heap: ensure that malloc(0) return NULL pointer in any poisoning configuration

components/heap/test/test_malloc.c

@@ -127,3 +127,10 @@ TEST_CASE("unreasonable allocs should all fail", "[heap]")
     TEST_ASSERT_NULL(test_malloc_wrapper(xPortGetFreeHeapSize() - 1));
 }
 
+TEST_CASE("malloc(0) should return a NULL pointer", "[heap]")
+{
+    void *p;
+    p = malloc(0);
+    TEST_ASSERT(p == NULL);
+}
+