Fix KeyError resulting from existing "string" argument
Fix KeyError resulting from existing "string" argument

_generateCurrentLineText may be called with an existing "string"
argument. Because we're potentially breaking the string up, based
on attributes such as language or formatting, this existing argument
is irrelevant. Plus calling voice() with two string arguments causes
a KeyError which interrupts the generation. Handle this condition by
removing the existing "string" argument before calling voice().

GNOME/orca,GNOME/orca,GNOME/orca,GNOME/orca