lavc/svq3: Do not write into memory defined as const.

Fixes a warning on ppc:
libavcodec/svq3.c:1055:21: warning: passing argument 1 of ‘av_write_bswap32’ discards 'const' qualifier from pointer target type

libavcodec/svq3.c

@@ -1048,12 +1048,12 @@ static int svq3_decode_slice_header(AVCodecContext *avctx)
         }
         memcpy(s->slice_buf, s->gb.buffer + s->gb.index / 8, slice_bytes);
 
-        init_get_bits(&s->gb_slice, s->slice_buf, slice_bits);
-
         if (s->watermark_key) {
-            uint32_t header = AV_RL32(&s->gb_slice.buffer[1]);
-            AV_WL32(&s->gb_slice.buffer[1], header ^ s->watermark_key);
+            uint32_t header = AV_RL32(&s->slice_buf[1]);
+            AV_WL32(&s->slice_buf[1], header ^ s->watermark_key);
         }
+        init_get_bits(&s->gb_slice, s->slice_buf, slice_bits);
+
         if (length > 0) {
             memmove(s->slice_buf, &s->slice_buf[slice_length], length - 1);
         }