libvpx: allow 0 as min quantizer value

Allow setting the min quantizer to 0 instead of 1 (libvpx allows 0); fixes #2136
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libvpxenc.c

@@ -285,7 +285,7 @@ static av_cold int vpx_init(AVCodecContext *avctx,
         }
     }
 
-    if (avctx->qmin > 0)
+    if (avctx->qmin >= 0)
         enccfg.rc_min_quantizer = avctx->qmin;
     if (avctx->qmax > 0)
         enccfg.rc_max_quantizer = avctx->qmax;