mmvideo: fix overreads of the input buffer.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mmvideo.c

@@ -123,11 +123,18 @@ static void mm_decode_intra(MmContext * s, int half_horiz, int half_vert, const
  */
 static void mm_decode_inter(MmContext * s, int half_horiz, int half_vert, const uint8_t *buf, int buf_size)
 {
-    const int data_ptr = 2 + AV_RL16(&buf[0]);
+    int data_ptr;
     int d, r, y;
+
+    if(buf_size < 2) {
+        av_log(s->avctx, AV_LOG_ERROR, "1 or less byte inter frame\n");
+        return;
+    }
+
+    data_ptr = 2 + AV_RL16(&buf[0]);
     d = data_ptr; r = 2; y = 0;
 
-    while(r < data_ptr) {
+    while(r + 1 < data_ptr) {
         int i, j;
         int length = buf[r] & 0x7f;
         int x = buf[r+1] + ((buf[r] & 0x80) << 1);
@@ -138,14 +145,19 @@ static void mm_decode_inter(MmContext * s, int half_horiz, int half_vert, const
             continue;
         }
 
-        if (y + half_vert >= s->avctx->height)
+        if (y + half_vert >= s->avctx->height || r+length > buf_size)
             return;
 
         for(i=0; i<length; i++) {
             for(j=0; j<8; j++) {
                 int replace = (buf[r+i] >> (7-j)) & 1;
                 if (replace) {
-                    int color = buf[d];
+                    int color;
+                    if (d >= buf_size) {
+                        av_log(s->avctx, AV_LOG_ERROR, "overread buf\n");
+                        return;
+                    }
+                    color = buf[d];
                     s->frame.data[0][y*s->frame.linesize[0] + x] = color;
                     if (half_horiz)
                         s->frame.data[0][y*s->frame.linesize[0] + x + 1] = color;