Crash when missing `here` in `[no Symbol here]` assertion.

[rbuckton]

Stack trace:

Error: Recovery failed to advance.
    at Parser.parseList (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:387:27)
    at Parser.parseNoSymbolHereAssertionTail (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:478:30)
    at Parser.parseAssertion (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:508:25)
    at Parser.parseSymbol (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:687:25)
    at Parser.parseSymbolSpanRest (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:709:29)
    at Parser.tryParseSymbolSpan (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:703:25)
    at Parser.parseSymbolSpanRest (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:710:27)
    at Parser.tryParseSymbolSpan (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:703:25)
    at Parser.parseRightHandSide (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:773:41)
    at Parser.parseElement (C:\path\to\project\node_modules\grammarkdown\dist\parser.js:242:29)