fix: Return original t in find_module if search_for_token fails

[lionkor]

Fixes #258.

The if block starting at line 400 in ldoc/parse.lua is the problem.
It seems to look for a module() call, and then:

• if it finds one, uses that for the module name
• if it doesn't, infers the name from the filename etc.

This means that in all modern Lua code it will infer the name, obviously.
The issue is that in line 402, it calls lang:find_module(tok, t, v) (that is Lua:find_module from lang.lua), which will (for anything that doesn't use module()) return nil, t, v where t, v is from self:search_for_token(tok, 'iden', 'module', t, v).

I know this is a bit to read, but here I believe the issue lies:

Lang:search_for_token(...) will return t as nil, here's why:

(from search_for_token)

   -- `t` may be nil here, which means it returns `false, nil, v`
   -- if `t` is nil
   return t ~= nil,t,v

And find_module says:

   res,t,v = self:search_for_token(tok,'iden','module',t,v)
   -- we know res may be `false` and `t` may be nil at the same time
   if not res then return nil,t,v end

We go back to parse.lua, line 402:

            module_found,t,v = lang:find_module(tok,t,v)

at this point module_found would be nil, and t would be nil also.

Now, a few lines down:

            if not t then
               F:warning('contains no items','warning',1)
               break;
            end -- run out of file!

So this is the issue - t may VERY well be nil.

Alternatively, we could:

• Remove the entire if statement that causes this warning, but Chestertons Fence speaks against this - I can't quite figure out exactly why this warning and the subsequent break need to be there, so I will not remove it.
• Remove the break only, and hope nothing relies on this - again, not sure that's the right move.

t arbitrarily being set to nil seems wrong here, so that's what I fixed, and it fixes the issue with the following reproducible sample:

--- my function
-- takes nothing and returns nothing
-- @return nothing
function DoNothing()
end

--
Thing = {
}

And also seems to work fine for code using module().

[lionkor]

Long story short - we preserve t across a function call that will set it to nil if it fails, because we expect that it may fail.