avfilter/vf_qp: Fix use of uinitialized variable

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

libavfilter/vf_qp.c

@@ -123,7 +123,6 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
                 double var_values[] = { !!in_qp_table, qp, x, y, s->qstride, s->h, 0};
                 static const char *var_names[] = { "known", "qp", "x", "y", "w", "h", NULL };
                 double temp_val;
-                int ret;
 
                 ret = av_expr_parse_and_eval(&temp_val, s->qp_expr_str,
                                             var_names, var_values,