While parsing a document Bind::asStringLiteral() Token::spell() was
called for a raw string literal token with a Token::literal nullptr.

This is due scanRawStringLiteral() not properly aborting for
invalid/incomplete code and that the code paths handling
multi-line-raw-strings were not limited to the highlighting case.

Address both cases.

Task-number: QTCREATORBUG-18941
Change-Id: I489d288ccbd7b59be396dada846613ff555436cf
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>