title: OpenCV findHomography errors
body: <p>I'm using the SURF feature detection of OpenCV to compare two images. 
  When I choose two identical images (sometimes the same image chosen), I get this:</p>
  
  <pre><code> OpenCV Error: Assertion failed (CV_IS_MAT(imagePoints) &amp;&amp; CV_IS_MAT(objectPoints)) in cvFindHomography, file /home/OpenCV/OpenCV-2.4.3/modules/calib3d/src/fundam.cpp, line 232 
  Qt has caught an exception thrown from an event handler. Throwing 
  exceptions from an event handler is not supported in Qt. You must 
  reimplement QApplication::notify() and catch all exceptions there. 
  
  terminate called after throwing an instance of 'cv::Exception' 
  what(): /home/OpenCV/OpenCV-2.4.3/modules/calib3d/src/fundam.cpp:232: error: (-215) CV_IS_MAT(imagePoints) &amp;&amp; CV_IS_MAT(objectPoints) in function cvFindHomography 
  
  Le programme s'est terminé subitement
  </code></pre>
  
  <p>A second execution of the code with two different images says that: </p>
  
  <pre><code>what(): /home/OpenCV/OpenCV-2.4.3/modules/calib3d/src/fundam.cpp:235: error: (-215) count &gt;= 4 in function cvFindHomography
  </code></pre>
  
  <p>I repeated the execution many times with differente images every time, and some test gave me the desired result. </p>