set pict_type to I type during init so parsers which dont set it get all i frames, fixes mp3 seeking

Originally committed as revision 8528 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/parser.c

@@ -70,6 +70,7 @@ AVCodecParserContext *av_parser_init(int codec_id)
         }
     }
     s->fetch_timestamp=1;
+    s->pict_type = FF_I_TYPE;
     return s;
 }