avcodec/mobiclip: Fix heap-buffer-overflow

The MobiClip decoder uses adjacent pixels for prediction; yet when
accessing the left pixel, it was forgotten to clip the x coordinate.
This results in an heap-buffer-overflow. It can e.g. be reproduced with
the sample from https://samples.ffmpeg.org/V-codecs/MOHD/crap.avi when
forcing the video decoder to mobiclip.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/mobiclip.c

@@ -905,7 +905,7 @@ static int predict_intra(AVCodecContext *avctx, AVFrame *frame, int ax, int ay,
             int arr1[16];
             int arr2[16];
             uint8_t *top = frame->data[plane] + FFMAX(ay - 1, 0) * frame->linesize[plane] + ax;
-            uint8_t *left = frame->data[plane] + ay * frame->linesize[plane] + ax - 1;
+            uint8_t *left = frame->data[plane] + ay * frame->linesize[plane] + FFMAX(ax - 1, 0);
             int bottommost = frame->data[plane][(ay + size - 1) * frame->linesize[plane] + FFMAX(ax - 1, 0)];
             int rightmost = frame->data[plane][FFMAX(ay - 1, 0) * frame->linesize[plane] + ax + size - 1];
             int avg = (bottommost + rightmost + 1) / 2 + 2 * get_se_golomb(gb);