ffmpeg: fix negative array index

Fixes CID733808
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_filter.c

@@ -37,7 +37,8 @@ enum AVPixelFormat choose_pixel_fmt(AVStream *st, AVCodec *codec, enum AVPixelFo
 {
     if (codec && codec->pix_fmts) {
         const enum AVPixelFormat *p = codec->pix_fmts;
-        int has_alpha= av_pix_fmt_desc_get(target)->nb_components % 2 == 0;
+        const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(target);
+        int has_alpha = desc ? desc->nb_components % 2 == 0 : 0;
         enum AVPixelFormat best= AV_PIX_FMT_NONE;
         if (st->codec->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL) {
             if (st->codec->codec_id == AV_CODEC_ID_MJPEG) {