bugfix: Use after free in fs.cpp's watch method (#568)

This use after free manifested intermittently when running lots and lots
of tests at a time in this PR:
#557.

The cause of this uaf happens because we do not close the `fs_event_t`
handle. When the `WatchHandle` wrapper gets closed, it invoked
`uv_event_stop` but not `uv_close`, so the memory associated with the
handle gets freed, but `libuv` doesn't know that this handle is closed.
This causes uv's internal data structures to get messed up and we may
accidentally touch this freed memory causing the Use-After-Free.

`WatchHandle::close` already invokes `uv_event_stop`, so we can remove
this from the implementation of `closeHandle`.

Author: Vighnesh-V

lute/fs/src/fs.cpp

@@ -561,6 +561,8 @@ struct WatchHandle
                 luaL_errorL(L, "Error stopping fs event: %s", uv_strerror(err));
             }
 
+            uv_close((uv_handle_t*) &handle, nullptr);
+
             isClosed = true;
 
             getRuntime(L)->releasePendingToken();
@@ -586,12 +588,6 @@ static int closeWatchHandle(lua_State* L)
         return 0;
     }
 
-    int err = uv_fs_event_stop(&handle->handle);
-    if (err)
-    {
-        luaL_errorL(L, "Error stopping fs event: %s", uv_strerror(err));
-    }
-
     handle->close();
 
     return 0;