sdpdemux: fix static code analyzer reported issues

static code analyzer reported:
sscanf format specification '%u' expects type 'unsigned int ' for 'u', but parameter 3 has a different type 'gint'
sscanf format specification '%u' expects type 'unsigned int ' for 'u', but parameter 4 has a different type 'gint'
sscanf format specification '%u' expects type 'unsigned int ' for 'u', but parameter 5 has a different type 'gint'