swscale/swscale_internal: Avoid unsigned for slice parameters

Mixing unsigned and signed often leads to unexpected arithmetic results.
Fixes: out of array write
Found-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale_internal.h

@@ -311,8 +311,8 @@ typedef struct SwsContext {
     int              nb_slice_ctx;
 
     // values passed to current sws_receive_slice() call
-    unsigned int dst_slice_start;
-    unsigned int dst_slice_height;
+    int dst_slice_start;
+    int dst_slice_height;
 
     /**
      * Note that src, dst, srcStride, dstStride will be copied in the