ScopeRunner::stop now returns if count is already negative

tests/io_test.cpp

@@ -39,6 +39,8 @@ int main() {
     assert(scope_runner.count == -1);
     auto lock = scope_runner.continue_lock();
     assert(!lock);
+    scope_runner.stop();
+    assert(scope_runner.count == -1);
 
     scope_runner.count = 0;
 

utility.hpp

@@ -289,6 +289,8 @@ namespace SimpleWeb {
     void stop() {
       long expected = 0;
       while(!count.compare_exchange_weak(expected, -1)) {
+        if(expected < 0)
+          return;
         expected = 0;
         spin_loop_pause();
       }