Terminate empty ASCII strings as well

src/value.cpp

@@ -410,7 +410,7 @@ namespace Exiv2 {
     int AsciiValue::read(const std::string& buf)
     {
         value_ = buf;
-        if (value_.size() > 0 && value_[value_.size()-1] != '\0') value_ += '\0';
+        if (value_.size() == 0 || value_[value_.size()-1] != '\0') value_ += '\0';
         return 0;
     }