nuv: dont try to copy an empty frame

Fixes assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/nuv.c

@@ -262,6 +262,7 @@ retry:
             av_log(avctx, AV_LOG_ERROR, "uncompressed frame too short\n");
             height = buf_size / c->width / 3 * 2;
         }
+        if(height > 0)
             copy_frame(&c->pic, buf, c->width, height);
         break;
     }