videoio(MSMF): remove "always true" check

modules/videoio/src/cap_msmf.cpp

@@ -1056,18 +1056,15 @@ HRESULT SourceReaderCB::Wait(DWORD dwMilliseconds, _ComPtr<IMFSample>& videoSamp
         return HRESULT_FROM_WIN32(GetLastError());
     }
 
-    if (!bEOS)
-    {
-        cv::AutoLock lock(m_mutex);
     bEOS = m_bEOS;
     if (!bEOS)
     {
+        cv::AutoLock lock(m_mutex);
         videoSample = m_lastSample;
         CV_Assert(videoSample);
         m_lastSample.Release();
         ResetEvent(m_hEvent);  // event is auto-reset, but we need this forced reset due time gap between wait() and mutex hold.
     }
-    }
 
     return m_hrStatus;
 }