Fix ScreenCapturerWinGdi to handle DesktopFrameWin::Create() errors.

DesktopFrameWin::Create() may return nullptr when it fails to allocate
windows bitmap. ScreenCapturerWinGdi wasn't handling that case properly.

BUG=527660

Review URL: https://codereview.webrtc.org/1309143007

Cr-Commit-Position: refs/heads/master@{#9865}

webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc

@@ -241,9 +241,10 @@ bool ScreenCapturerWinGdi::CaptureImage() {
         DesktopFrame::kBytesPerPixel;
     SharedMemory* shared_memory = callback_->CreateSharedMemory(buffer_size);
 
-    rtc::scoped_ptr<DesktopFrame> buffer;
-    buffer.reset(
+    rtc::scoped_ptr<DesktopFrame> buffer(
         DesktopFrameWin::Create(size, shared_memory, desktop_dc_));
+    if (!buffer.get())
+      return false;
     queue_.ReplaceCurrentFrame(buffer.release());
   }