avcodec/fraps: Fix segfault with negative linesizes

Using unsigned and negative linesizes doesn't really work.
Use ptrdiff_t instead. This fixes the fraps-v0 and fraps-v1
FATE tests with negative linesizes.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/fraps.c

@@ -141,7 +141,6 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *f,
     int buf_size           = avpkt->size;
     uint32_t header;
     unsigned int version,header_size;
-    unsigned int x, y;
     const uint32_t *buf32;
     uint32_t *luma1,*luma2,*cb,*cr;
     uint32_t offs[4];
@@ -238,12 +237,12 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *f,
         }
 
         buf32 = (const uint32_t*)buf;
-        for (y = 0; y < avctx->height / 2; y++) {
+        for (ptrdiff_t y = 0; y < avctx->height / 2; y++) {
             luma1 = (uint32_t*)&f->data[0][  y * 2      * f->linesize[0] ];
             luma2 = (uint32_t*)&f->data[0][ (y * 2 + 1) * f->linesize[0] ];
             cr    = (uint32_t*)&f->data[1][  y          * f->linesize[1] ];
             cb    = (uint32_t*)&f->data[2][  y          * f->linesize[2] ];
-            for (x = 0; x < avctx->width; x += 8) {
+            for (ptrdiff_t x = 0; x < avctx->width; x += 8) {
                 *luma1++ = *buf32++;
                 *luma1++ = *buf32++;
                 *luma2++ = *buf32++;
@@ -258,18 +257,18 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *f,
         if (is_pal) {
             uint32_t *pal = (uint32_t *)f->data[1];
 
-            for (y = 0; y < 256; y++) {
+            for (unsigned y = 0; y < 256; y++) {
                 pal[y] = AV_RL32(buf) | 0xFF000000;
                 buf += 4;
             }
 
-            for (y = 0; y <avctx->height; y++)
+            for (ptrdiff_t y = 0; y < avctx->height; y++)
                 memcpy(&f->data[0][y * f->linesize[0]],
                        &buf[y * avctx->width],
                        avctx->width);
         } else {
         /* Fraps v1 is an upside-down BGR24 */
-            for (y = 0; y<avctx->height; y++)
+            for (ptrdiff_t y = 0; y < avctx->height; y++)
                 memcpy(&f->data[0][(avctx->height - y - 1) * f->linesize[0]],
                        &buf[y * avctx->width * 3],
                        3 * avctx->width);