Change return type on File::asZString.

File.cpp

@@ -82,9 +82,8 @@ namespace coreutils {
       return std::string(buffer, size);
    }
 
-   coreutils::ZString & File::asZString() {
-      zstring = ZString(buffer, size);
-      return zstring;
+   coreutils::ZString File::asZString() {
+      return ZString(buffer, size);
    }
 
    bool File::eof() {