ffprobe: only decode frame if the codec is available

Fix crash when a stream codec is unknown, in particular fix trac
ticket #1243.

ffprobe.c

@@ -1661,6 +1661,7 @@ static av_always_inline int get_decoded_frame(AVFormatContext *fmt_ctx,
     int ret = 0;
 
     *got_frame = 0;
+    if (dec_ctx->codec) {
     switch (dec_ctx->codec_type) {
     case AVMEDIA_TYPE_VIDEO:
         ret = avcodec_decode_video2(dec_ctx, frame, got_frame, pkt);
@@ -1670,6 +1671,7 @@ static av_always_inline int get_decoded_frame(AVFormatContext *fmt_ctx,
         ret = avcodec_decode_audio4(dec_ctx, frame, got_frame, pkt);
         break;
     }
+    }
 
     return ret;
 }