mpegaudioparse: Fix compilation with gcc 7

mpegaudioparse: Fix compilation with gcc 7

gstmpegaudioparse.c: In function ‘gst_mpeg_audio_parse_reset’:
gstmpegaudioparse.c:209:3: error: ‘memset’ used with length equal to number of elements without multiplication by element size [-Werror=memset-elt-size]
   memset (mp3parse->xing_seek_table_inverse, 0, 256);
   ^~~~~~
gstmpegaudioparse.c: In function ‘gst_mpeg_audio_parse_handle_first_frame’:
gstmpegaudioparse.c:951:7: error: ‘memset’ used with length equal to number of elements without multiplication by element size [-Werror=memset-elt-size]
       memset (mp3parse->xing_seek_table_inverse, 0, 256);
       ^~~~~~