vf_drawtext: add missing clear of pointers after av_expr_free()

Fixes segfault when using sendcmd with drawtext.

Since LIBAVFILTER_VERSION_MAJOR 5 FF_API_DRAWTEXT_OLD_TIMELINE
evaluates to 0.
Signed-off-by: Karl Kiniger <karl.kiniger@med.ge.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_drawtext.c

@@ -716,6 +716,8 @@ static av_cold void uninit(AVFilterContext *ctx)
 #if FF_API_DRAWTEXT_OLD_TIMELINE
     av_expr_free(s->draw_pexpr);
     s->x_pexpr = s->y_pexpr = s->draw_pexpr = NULL;
+#else
+    s->x_pexpr = s->y_pexpr = NULL;
 #endif
     av_freep(&s->positions);
     s->nb_positions = 0;