io: fix possible double free of task error object

If a QIOTask has an error set and the calling code uses
qio_task_propagate_error() to steal the reference to
that Error object, the task would not clear its own
reference. This would lead to a double-free when
qio_task_free runs, if the caller had (correctly) freed
the Error object they now owned.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

io/task.c

@@ -157,6 +157,7 @@ bool qio_task_propagate_error(QIOTask *task,
 {
     if (task->err) {
         error_propagate(errp, task->err);
+        task->err = NULL;
         return true;
     }
 

tests/test-io-task.c

@@ -127,7 +127,7 @@ static void test_task_failure(void)
     g_assert(data.source == obj);
     g_assert(data.err == err);
     g_assert(data.freed == false);
-
+    error_free(data.err);
 }
 
 
@@ -238,6 +238,8 @@ static void test_task_thread_failure(void)
     g_assert(data.source == obj);
     g_assert(data.err != NULL);
 
+    error_free(data.err);
+
     self = g_thread_self();
 
     /* Make sure the test_task_thread_worker actually got