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>