Additional checks for strange num_val in FLV metadata

Avoids some "Unsupported audio codec (6)" message in FLVs, e.g.
        Example of such problematic bitstream is 'bad_codec6.flv'
        in ftp's /incoming directory.

Originally committed as revision 12510 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/flvdec.c

@@ -175,9 +175,11 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream, AVStream *vst
             if(!strcmp(key, "duration")) s->duration = num_val * AV_TIME_BASE;
 //            else if(!strcmp(key, "width")  && vcodec && num_val > 0) vcodec->width  = num_val;
 //            else if(!strcmp(key, "height") && vcodec && num_val > 0) vcodec->height = num_val;
-            else if(!strcmp(key, "audiocodecid") && acodec) flv_set_audio_codec(s, astream, (int)num_val << FLV_AUDIO_CODECID_OFFSET);
-            else if(!strcmp(key, "videocodecid") && vcodec) flv_set_video_codec(s, vstream, (int)num_val);
-            else if(!strcmp(key, "audiosamplesize") && acodec && num_val >= 0) {
+            else if(!strcmp(key, "audiocodecid") && acodec && 0 <= (int)num_val)
+                flv_set_audio_codec(s, astream, (int)num_val << FLV_AUDIO_CODECID_OFFSET);
+            else if(!strcmp(key, "videocodecid") && vcodec && 0 <= (int)num_val)
+                flv_set_video_codec(s, vstream, (int)num_val);
+            else if(!strcmp(key, "audiosamplesize") && acodec && 0 < (int)num_val) {
                 acodec->bits_per_sample = num_val;
                 //we may have to rewrite a previously read codecid because FLV only marks PCM endianness.
                 if(num_val == 8 && (acodec->codec_id == CODEC_ID_PCM_S16BE || acodec->codec_id == CODEC_ID_PCM_S16LE))