Use size() instead of getLength()

For GooString; see the poppler commit:

https://gitlab.freedesktop.org/poppler/poppler/-/commit/f6dcd06445b6b7795b4c8f654206c5aef3d79100

Author: StevenMaude

src/main.cpp

@@ -203,12 +203,12 @@ void pack_stream_content(Stream *stream) {
   stream->fillGooString(&content);
   stream->close();
 
-  packer.pack_bin(content.getLength());
-  packer.pack_bin_body(content.toStr().c_str(), content.getLength());
+  packer.pack_bin(content.size());
+  packer.pack_bin_body(content.toStr().c_str(), content.size());
 }
 
 void pack_string(const GooString *string) {
-  if (!string || string->getLength() <= 0)
+  if (!string || string->size() <= 0)
     packer.pack_nil();
   else
     packer.pack(string->toStr());