vf_drawtext: Fix memory leak

libavfilter/vf_drawtext.c

@@ -709,7 +709,8 @@ static av_cold void uninit(AVFilterContext *ctx)
 
     av_expr_free(s->x_pexpr);
     av_expr_free(s->y_pexpr);
-    s->x_pexpr = s->y_pexpr = NULL;
+    av_expr_free(s->a_pexpr);
+    s->x_pexpr = s->y_pexpr = s->a_pexpr = NULL;
     av_freep(&s->positions);
     s->nb_positions = 0;
 
@@ -752,7 +753,8 @@ static int config_input(AVFilterLink *inlink)
 
     av_expr_free(s->x_pexpr);
     av_expr_free(s->y_pexpr);
-    s->x_pexpr = s->y_pexpr = NULL;
+    av_expr_free(s->a_pexpr);
+    s->x_pexpr = s->y_pexpr = s->a_pexpr = NULL;
 
     if ((ret = av_expr_parse(&s->x_pexpr, s->x_expr, var_names,
                              NULL, NULL, fun2_names, fun2, 0, ctx)) < 0 ||