subparse: don't reset allowed tags

subparse: don't reset allowed tags

When a discont buffer is processed, the state is re-initialized, which
nullifies the allowed_tags.

The problem is when a subrip string with tags is processed and allowed_tags is
NULL. The function subrip_unescape_formatting() calls g_strjoinv with a
str_array as NULL, leading to a GLib-CRITICAL.

This patch removes the allowed_tags resetting, in parser_state_init(), but
move it into gst_sub_parse_format_autodetect().

https://bugzilla.gnome.org/show_bug.cgi?id=768525