swscale/output: silence warning about undefined 'ASSERT_LEVEL'

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libswscale/output.c

@@ -1152,7 +1152,7 @@ yuv2rgb_write(uint8_t *_dest, int i, int Y1, int Y2,
             dest[i * 2 + 0] = r[Y1] + g[Y1] + b[Y1] + (A1 << sh);
             dest[i * 2 + 1] = r[Y2] + g[Y2] + b[Y2] + (A2 << sh);
         } else {
-#if ASSERT_LEVEL > 1
+#if defined(ASSERT_LEVEL) && ASSERT_LEVEL > 1
             int sh = (target == AV_PIX_FMT_RGB32_1 || target == AV_PIX_FMT_BGR32_1) ? 0 : 24;
 
             av_assert2((((r[Y1] + g[Y1] + b[Y1]) >> sh) & 0xFF) == 0xFF);