Fix possible infinite loop in TaskScheduler.process.

Fixes a discrepancy in TaskFiberQueue between empty and length, which causes process() to never return, thus processing events without timeout indefinitely.

source/vibe/core/task.d

@@ -962,9 +962,29 @@ private struct TaskFiberQueue {
 		task.m_queue = null;
 		task.m_prev = null;
 		task.m_next = null;
+		length--;
 	}
 }
 
+unittest {
+	auto f1 = new TaskFiber;
+	auto f2 = new TaskFiber;
+
+	TaskFiberQueue q;
+	assert(q.empty && q.length == 0);
+	q.insertFront(f1);
+	assert(!q.empty && q.length == 1);
+	q.insertFront(f2);
+	assert(!q.empty && q.length == 2);
+	q.popFront();
+	assert(!q.empty && q.length == 1);
+	q.popFront();
+	assert(q.empty && q.length == 0);
+	q.insertFront(f1);
+	q.remove(f1);
+	assert(q.empty && q.length == 0);
+}
+
 private struct FLSInfo {
 	void function(void[], size_t) fct;
 	size_t offset;