blobtrack_sample FGMask video assertion (Bug #924)

More simple sample for generating this problem:

 1#include "opencv2/video/video.hpp" 
 2#include "opencv2/highgui/highgui.hpp" 
 3#include <opencv2/imgproc/imgproc.hpp>
 4#include <stdio.h>
 5#include <iostream>
 6using namespace cv;
 7using namespace std;
 8int main(int argc, char* argv[])
 9{   /* Main function: */
10    CvCapture* pCap = NULL;
11    char* avi_name = argv[1];
12    printf("Video name = %s\n", avi_name);
13    /* Create source video: */
14    if(avi_name)
15        pCap = cvCaptureFromFile(avi_name);
16
17    if(pCap==NULL)
18    {
19        printf("\nCan't open %s file\n",avi_name);
20        return -1;
21    }
22
23    cvNamedWindow("gray");
24
25    CvVideoWriter* pFGAvi = NULL;
26
27    while(pCap)
28    {   /* Main loop: */
29        IplImage*   pImg  = NULL;
30        pImg = cvQueryFrame(pCap);
31        if(pImg == NULL) break;
32
33        if (pFGAvi == NULL)
34           pFGAvi=cvCreateVideoWriter(
35            argv[2],
36            CV_FOURCC('x','v','i','d'),
37            25,
38            cvSize(pImg->width, pImg->height),1);
39
40        IplImage* gray = cvCreateImage(cvSize(pImg->width, pImg->height),pImg->depth,1);
41        cvCvtColor(pImg, gray, CV_BGR2GRAY);
42
43        IplImage* gray3 = cvCreateImage(cvSize(pImg->width, pImg->height),pImg->depth,3);
44        cvCvtColor(gray, gray3, CV_GRAY2BGR);
45
46        cvWriteFrame(pFGAvi, gray3);
47
48        cvShowImage("gray", gray);
49        cvWaitKey(1);
50
51        if (gray)
52            cvReleaseImage(&gray);
53        if (gray3)
54            cvReleaseImage(&gray3);
55    }
56
57    if (pFGAvi)
58        cvReleaseVideoWriter(&pFGAvi);
59
60    if(pCap)
61        cvReleaseCapture(&pCap);
62
63    pCap = cvCaptureFromFile(argv[2]);
64
65    if(pCap==NULL)
66    {
67        printf("\nCan't open %s file\n",avi_name);
68        return -1;
69    }
70
71    while(pCap)
72    {   /* Main loop: */
73        IplImage*   pImg  = NULL;
74        pImg = cvQueryFrame(pCap);
75        if(pImg == NULL) break;
76        cvShowImage("gray", pImg);
77        cvWaitKey(1);
78    }
79
80    if(pCap)
81        cvReleaseCapture(&pCap);
82    return 0;
83}   /* main() */