#1711 [PATCH] Fix highlighting empty double-quoted string (``) in cpp lexer

[Jiri Techet]

See the patch for more info.

The sc.Forward() is performed once more at the end of the
function and performing it here causes the second ` is skipped in ``.

Probably caused by copy-pasting the code above and not realizing
this one reads just a single character.

[Neil Hodgson]

Fix committed as [a00dab].