qpeg: fix an off by 1 error in the MV check

height - me_y is the line from which we read, so it must be strictly
smaller than the frame height. Fixes possible invalid reads in corrupted
files.

Also, use a proper context for logging the error.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

libavcodec/qpeg.c

@@ -161,9 +161,9 @@ static void qpeg_decode_inter(QpegContext *qctx, uint8_t *dst,
 
                     /* check motion vector */
                     if ((me_x + filled < 0) || (me_x + me_w + filled > width) ||
-                       (height - me_y - me_h < 0) || (height - me_y > orig_height) ||
+                       (height - me_y - me_h < 0) || (height - me_y >= orig_height) ||
                        (filled + me_w > width) || (height - me_h < 0))
-                        av_log(NULL, AV_LOG_ERROR, "Bogus motion vector (%i,%i), block size %ix%i at %i,%i\n",
+                        av_log(qctx->avctx, AV_LOG_ERROR, "Bogus motion vector (%i,%i), block size %ix%i at %i,%i\n",
                                me_x, me_y, me_w, me_h, filled, height);
                     else {
                         /* do motion compensation */