[fake-display] Only send VSync with valid ConfigStamps.

Bug: 401388764
Test: fx test //src/graphics/display
Change-Id: Ibadf7465f60aaa262bc93fbcbfdcf415fe9194be
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1235344
Reviewed-by: Yilong Li <[email protected]>
Fuchsia-Auto-Submit: Victor Costan <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

Author: pwnall

src/graphics/display/drivers/fake/fake-display.cc

@@ -974,6 +974,11 @@ void FakeDisplay::SendVsync() {
     std::lock_guard lock(mutex_);
     vsync_config_stamp = applied_config_stamp_;
   }
+  if (vsync_config_stamp == display::kInvalidDriverConfigStamp) {
+    // No configuration was applied yet.
+    return;
+  }
+
   const config_stamp_t banjo_vsync_config_stamp =
       display::ToBanjoDriverConfigStamp(vsync_config_stamp);