implemented exception of unlink a capture in case of non-KF

src/capture/capture_base.cpp

@@ -291,7 +291,11 @@ void CaptureBase::setCalibration(const VectorXs& _calib)
 
 void CaptureBase::link(FrameBasePtr _frm_ptr)
 {
-    assert(this->getFrame() == nullptr && "linking an already linked capture");
+    assert((this->getFrame() == nullptr || !this->getFrame()->isKey()) && "linking a capture already linked to a KF");
+
+    // unlink from previous non-key frame
+    if (this->getFrame())
+        this->getFrame()->removeCapture(shared_from_this());
 
     if(_frm_ptr)
     {