insufficient thread locking around avcodec_open/close() (Bug #3750)

Hi everyone.
I've been struggling with insufficient thread locking issues lately. I have an application that is supposed to read two different *.mp4 files and an image file.
Then the idea is to take frames from each video capture stream and place these frames at two different locations on the canvas (the image file I mentioned earlier).
There is only one shared object between the two threads and the main, that is the canvas image.
Below is the source code to accomplish this task, I would appreciate if anyone could point out what I'm doing wrong or what is wrong with this setup.

#include <iostream>
#include "opencv2/highgui/highgui.hpp" 
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <string>
#include <thread>
#include <mutex>

using namespace std;
using namespace cv;
//The main canvas (a background image)
Mat canvas=imread("canvas.jpg", CV_LOAD_IMAGE_COLOR);
// the mutex for accessing the canvas variable
mutex mu;
// The function that plays a video on a predefined ROI on the canvas
void play(string n, Point p)  //this function could be called n number of times as its own thread
  {
    VideoCapture cap(n);  //open a local video file 
    int fps=cap.get(CV_CAP_PROP_FPS); // get it's frames per-seconds frame rates
    char k; // for exiting the frame fetching loop
    Mat temp;  // a temporary holding variable for each frame from the stream
    while(1)
      {
        unique_lock<mutex> locker(mu);  // lock the mutex (shared memory is the canvas matrix)
        cap>>temp;  // get a frame from the stream to the temporary buffer
        if(temp.empty())// if stream is empty get out of this loop
          { 
              locker.unlock();  // unlock the mutex first before exiting this loop
              break;  // exit the loop now.
          }
        // syntax= src_image.copyto(dst_image(Rect(Point(x,y), size(src_image))))
        //dest_image is the shared object between the threads.
        temp.copyTo(canvas(Rect(p.x, p.y, temp.cols, temp.rows)));  // past a frame onto the canvas at location (x,y) and size(of the frame)
        locker.unlock();// done interacting with the shared variable, so unlock the mutex
      }
  }
int main()
  {
      char k; // for breaking out of the main loop ( the display image loop)
      namedWindow("MainWindow", WINDOW_AUTOSIZE); // create the main window
      // create and start the first thread, pass a filename and a coordinates to place the frame on the canvas
      thread P1(play, "video-copy.mp4", Point(0,0));
      // create and start the second thread, they should both be able to access the canvas variable (one at a time ofcourse)
      thread P2(play, "video.mp4", Point(200,300));
      while(1) // keep displaying the canvas as it is changing

            unique_lock<mutex> locker(mu);  // lock the mutex again, because we are using the canvas (shared object) again,
            imshow("MainWindow", canvas); // show image
            locker.unlock(); // unlock the mutex again.
            k=waitKey(10);   // flush the buffer and wait for 10 milliseconds
            if(k==27) //exit the display image loop.
                break;
        }
      // Join the child threads back with the parrent thread before exiting.
      P1.join();
      P2.join();
      //and finally
      destroyWindow("MainWindow"); // destroy the main window
      return 0; // end the main.
  }
/*   The errors I always get but cant find what is causing it

(MainWindow:3157): GLib-GObject-CRITICAL **: g_object_remove_weak_pointer: assertion `G_IS_OBJECT (object)' failed

[NULL @ 0xb1b06ca0] insufficient thread locking around avcodec_open/close()
[NULL @ 0xb1b07aa0] insufficient thread locking around avcodec_open/close()
[NULL @ 0xb1b06ca0] insufficient thread locking around avcodec_open/close()
[NULL @ 0xb1b07aa0] insufficient thread locking around avcodec_open/close()
[IMGUTILS @ 0xb28fde64] Picture size 0x0 is invalid
[IMGUTILS @ 0xb28fde94] Picture size 0x0 is invalid
[swscaler @ 0xad180300] bad dst image pointers

*/