screencopy: fix read on incorrect monitor render

src/protocols/Screencopy.cpp

@@ -297,6 +297,9 @@ void CScreencopyProtocolManager::onRenderEnd(CMonitor* pMonitor) {
             continue;
         }
 
+        if (f->pMonitor != pMonitor)
+            continue;
+
         shareFrame(f);
 
         framesToRemove.push_back(f);
@@ -408,4 +411,4 @@ bool CScreencopyProtocolManager::copyFrameDmabuf(SScreencopyFrame* frame) {
     // todo
     Debug::log(ERR, "DMABUF copying not impl'd!");
     return false;
-}
+}