Don't report current text in parser error messages

Don't report current text in parser error messages

The values of `yytext` and `yyleng` might not be properly defined when
the error function is called (in particular if the lexer reached EOF).
While this might just cause non-printable characters in the output, it
could actually lead to a crash depending on where `yytext` points.

Closes strongswan/strongswan#346.