libvpxenc: allow qmax of 0

this is valid for both vp8 & vp9 and necessary for lossless in the
latter
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libvpxenc.c

@@ -320,7 +320,7 @@ static av_cold int vpx_init(AVCodecContext *avctx,
 
     if (avctx->qmin >= 0)
         enccfg.rc_min_quantizer = avctx->qmin;
-    if (avctx->qmax > 0)
+    if (avctx->qmax >= 0)
         enccfg.rc_max_quantizer = avctx->qmax;
 
     if (enccfg.rc_end_usage == VPX_CQ) {