utvideodec: Handle slice_height being zero

Fixes out of array accesses.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Bug-Id: CVE-2014-9604
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/utvideodec.c

@@ -213,6 +213,8 @@ static void restore_median(uint8_t *src, int step, int stride,
         slice_start  = ((slice * height) / slices) & cmask;
         slice_height = ((((slice + 1) * height) / slices) & cmask) -
                        slice_start;
+        if (!slice_height)
+            continue;
 
         bsrc = src + slice_start * stride;
 
@@ -269,6 +271,8 @@ static void restore_median_il(uint8_t *src, int step, int stride,
         slice_height   = ((((slice + 1) * height) / slices) & cmask) -
                          slice_start;
         slice_height >>= 1;
+        if (!slice_height)
+            continue;
 
         bsrc = src + slice_start * stride;