From Philipp Siemoleit,

There are some group codes (i.e. "62") which are interpreted as
    dxfDataType::SHORT. That's right because the dxf-specification defines
    "16 bit integer" as the type for the corresponding value.
    But readerBase::readGroup() calls readValue(std::ifstream&, unsigned
    short). I changed readValue(std::ifstream&, unsigned short) to
    readValue(std::ifstream&, short). I found no group code at the dxf-specs
    which needs a "16 bit unsigned integer" value. So the
    readValue(std::ifstream&, unsigned short) function is obsolete - right?

src/osgPlugins/dxf/dxfReader.cpp

@@ -124,11 +124,11 @@ bool readerText::readValue(std::ifstream& f, bool &b)
         return false;
     }
 }
-bool readerText::readValue(std::ifstream& f, unsigned short &s)
+bool readerText::readValue(std::ifstream& f, short &s)
 {
     if (getTrimmedLine(f)) {
         _str >> s;
-        return success(!_str.fail(), "unsigned short");
+        return success(!_str.fail(), "short");
     } else {
         return false;
     }