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
AnnotFreeText::generateFreeTextAppearance.
diff --git a/poppler/Annot.cc b/poppler/Annot.cc
index bbccd82..0bf9864 100644
--- a/poppler/Annot.cc
+++ b/poppler/Annot.cc
@@ -753,9 +753,10 @@
     }
     if (i >= 2) {
       // We are expecting a name, therefore the first letter should be '/'.
-      if ((*daToks)[i-2] && ((const char*)((*daToks)[i-2]))[0] == '/') {
+      const GooString* fontToken = (*daToks)[i-2];
+      if (fontToken && fontToken->getLength() > 1 && fontToken->getChar(0) == '/') {
         // The +1 is here to skip the leading '/'.
-        fontName = Object(objName, ((const char*)(*daToks)[i-2])+1);
+        fontName = Object(objName, fontToken->c_str() + 1);
       }
     }
     // Scan backwards: we are looking for the last set value