Fix bad cast of GooString* to const char* in DefaultAppearance

Casting GooString* to const char* was probably a reminder from a time
where GooString layout had been under poppler control.
Nowadays GooString derives from std::string and we must not rely upon
the memory layout.

The bug leads to DefaultAppearance::getFontName() always returning an
invalid Object. The offending code is reachable via
1 file changed