videoio: fixed segmentation fault at frame size change

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -750,9 +750,6 @@ bool CvCapture_FFMPEG::retrieveFrame(int, unsigned char** data, int* step, int*
     if( !video_st || !picture->data[0] )
         return false;
 
-    avpicture_fill((AVPicture*)&rgb_picture, rgb_picture.data[0], AV_PIX_FMT_RGB24,
-                   video_st->codec->width, video_st->codec->height);
-
     if( img_convert_ctx == NULL ||
         frame.width != video_st->codec->width ||
         frame.height != video_st->codec->height )
@@ -775,8 +772,17 @@ bool CvCapture_FFMPEG::retrieveFrame(int, unsigned char** data, int* step, int*
 
         if (img_convert_ctx == NULL)
             return false;//CV_Error(0, "Cannot initialize the conversion context!");
+
+        rgb_picture.data[0] = (uint8_t*)realloc(rgb_picture.data[0],
+                avpicture_get_size( AV_PIX_FMT_BGR24,
+                                    video_st->codec->width, video_st->codec->height ));
+        frame.data = rgb_picture.data[0];
     }
 
+    avpicture_fill((AVPicture*)&rgb_picture, rgb_picture.data[0], AV_PIX_FMT_RGB24,
+                   video_st->codec->width, video_st->codec->height);
+    frame.step = rgb_picture.linesize[0];
+
     sws_scale(
             img_convert_ctx,
             picture->data,