lavfi/vf_vpp_qsv: check output format string against NULL pointer

This is in preparation for reusing the code for other QSV filters. E.g.
deinterlacing_qsv may have an option array without format option

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavfilter/vf_vpp_qsv.c

@@ -279,7 +279,7 @@ static av_cold int vpp_init(AVFilterContext *ctx)
 {
     VPPContext  *vpp  = ctx->priv;
 
-    if (!strcmp(vpp->output_format_str, "same")) {
+    if (!vpp->output_format_str || !strcmp(vpp->output_format_str, "same")) {
         vpp->out_format = AV_PIX_FMT_NONE;
     } else {
         vpp->out_format = av_get_pix_fmt(vpp->output_format_str);