vf_drawbox: give all v_log() a context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_drawbox.c

@@ -190,7 +190,7 @@ static int config_input(AVFilterLink *inlink)
     return 0;
 
 fail:
-    av_log(NULL, AV_LOG_ERROR,
+    av_log(ctx, AV_LOG_ERROR,
            "Error when evaluating the expression '%s'.\n",
            expr);
     return ret;