Fix crash when parsing karaoke data from a line without an extra field

[line0]

Passing a line table without an extra field to aegisub.parse_karaoke_data(line), e.g. via the public karaskel.preproc_line_text(meta, styles, line) API, immediately crashes Aegisub (at least on Windows).

The crash is caused by a bug in the LuaAssFile::LuaToAssEntry() function, whose dialog branch leaves a nil on the stack when the extra field is absent or nil. With the now unbalanced stack, LuaAssFile::LuaParseKaraokeData() ends up trying to write syllables into a nil instead of the line table expected at that stack index.
Fixed by popping the value in that case, so the stack remains balanced and the line table at its the expected index.

None of the other LuaAssFile::LuaToAssEntry() call sites are affected either way.

Reproduction

Any automation script, with a subtitle file open:

aegisub.parse_karaoke_data{
  class = "dialogue", comment = false, layer = 0,
  start_time = 0, end_time = 5000, style = "Default", actor = "",
  margin_l = 0, margin_r = 0, margin_t = 0, effect = "", text = "Hello world",
}

The bug is not triggered when the lines passed to karaskel.preproc_line_text(meta, styles, line) are directly taken from the subtitles object, as that is always initialized with an empty extradata table.

However it is easy to hit when a script builds its own line tables, particularly when relying on Aegisub's own documentation:

• automation/v4-docs/subtitle-data.txt gives the signature as aegisub.parse_karaoke_data(text) with @text (string), but the function takes a dialogue line table and errors on a string.
• Neither that file nor the karaskel page on aegisub.org lists extra among a dialogue line's fields, so the documented set of fields is exactly the set that crashes.

Side note

The comment above LuaAssFile::LuaToAssEntry() claims to pop the table from the stack, but not only does it not, LuaAssFile::LuaParseKaraokeData() also depends on it not doing so.

[CoffeeFlux]

The tests are pretty specific to libaegisub, so this is a layering violation, no? I'll try to get aegisub cli in soon, which will make this a lot easier.