Crash with multiple languages in `spelllang`.

[kevincox]

Error detected while processing function grammarous#check_current_buffer[23]..<SNR>81_invoke_check:       
line   39:
E121: Undefined variable: lang
E116: Invalid arguments for function printf
E15: Invalid expression: printf(   '-c %s -l %s --api %s',   &fileencoding ? &fileencoding : &encoding,   lang,   s
ubstitute(tmpfile, '\\\s\@!', '\\\\', 'g') )
line   48:
E121: Undefined variable: cmdargs
E15: Invalid expression: '-d ' . join(disabled_rules, ',') . ' ' . cmdargs
line   67:
E121: Undefined variable: cmdargs
E116: Invalid arguments for function printf
E15: Invalid expression: printf('%s %s', g:grammarous#languagetool_cmd, cmdargs)
line   86:
E121: Undefined variable: cmd
E116: Invalid arguments for function jobstart
E15: Invalid expression: jobstart(cmd, opts)
line   87:
E121: Undefined variable: job

This appears to occur because there is a path through the function where lang does not get defined:

vim-grammarous/autoload/grammarous.vim

Lines 275 to 284 in db46357

	if g:grammarous#use_vim_spelllang
	" Convert vim spelllang to languagetool spelllang
	if len(split(&spelllang, '_')) == 1
	let lang = split(&spelllang, '_')[0]
	elseif len(split(&spelllang, '_')) == 2
	let lang = split(&spelllang, '_')[0].'-'.toupper(split(&spelllang, '_')[1])
	endif
	else
	let lang = a:0 == 1 ? g:grammarous#default_lang : a:1
	endif

It appears that there should be a fallback for =0 (use default_lang?) and >2 segments (raise an error?).

This problem seems to be occurring because I have multiple languages listed in my spelllang which is allowed.

:echo &spelllang
en_ca,en_us,en
:echo split(&spelllang, '_')
['en', 'ca,en', 'en']

I think the best solution is to only consider the first element of spelllang.