avcodec/utvideodec: fix assumtation that slice_height >= 1

Fixes out of array read
Fixes: asan_heap-oob_2573085_3783_utvideo_rgba_median.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utvideodec.c

@@ -224,7 +224,7 @@ static void restore_median(uint8_t *src, int step, int stride,
             A        = bsrc[i];
         }
         bsrc += stride;
-        if (slice_height == 1)
+        if (slice_height <= 1)
             continue;
         // second line - first element has top prediction, the rest uses median
         C        = bsrc[-stride];
@@ -284,7 +284,7 @@ static void restore_median_il(uint8_t *src, int step, int stride,
             A                 = bsrc[stride + i];
         }
         bsrc += stride2;
-        if (slice_height == 1)
+        if (slice_height <= 1)
             continue;
         // second line - first element has top prediction, the rest uses median
         C        = bsrc[-stride2];