Crash in DirectX capturer

A tiny but critical change to avoid a crash failure in DirectX capturer.
A good news is this failure is caught by ScreenCapturer integration tests.

BUG=314516

Review-Url: https://codereview.webrtc.org/2494893002
Cr-Original-Commit-Position: refs/heads/master@{#15046}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 307409681680d9c783b4de1b48121a3056acfaf7
1 file changed