Dangling references in lgf_reader_writer_test.cc

[Peter Madarasi]

Using g++ 9.4.0, make check fails with the error below.

The following tests FAILED:
         24 - lgf_reader_writer_test (Child aborted)
Errors while running CTest
make[3]: *** [CMakeFiles/check.dir/build.make:57: CMakeFiles/check] Error 8
make[2]: *** [CMakeFiles/Makefile2:321: CMakeFiles/check.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:328: CMakeFiles/check.dir/rule] Error 2
make: *** [Makefile:210: check] Error 2
terminate called after throwing an instance of 'lemon::FormatError'
  what():  lemon:FormatError: Item not found
Aborted (core dumped)

The reason seems to be dangling references in the test. The error was caused by passing a temporary arc returned by graph.direct(e1, false) as a reference. I also fixed the handling of temporarily created functors ReaderConverter and WriterConverter.

The attached patch fixes the issue.